To start, read the official release notes.
If your install fits into "vanilla Ubuntu plus maybe a handful of 3rd-party repos", then this guide for a simple upgrade to Ubuntu 24.04 "Noble Numbat" from Ubuntu 22.04 "Jammy Jellyfish" can be helpful. 3rd-party repos are handled with a find command.
Note upgrade is only supported from Ubuntu 22.04 or later to Ubuntu 24.04. If you are on Ubuntu 20.04, upgrade to Ubuntu 22.04 first. Then once on Ubuntu 22.04, you can upgrade to Ubuntu 24.04.
- Check free disk space
 
df -h
5 GiB free is a conservative amount. sudo apt clean and sudo apt autoremove can be used to free some disk space.
- 
Identify any 3rd-party repos that may need to be updated. They'll be changed with a
findcommand.ls /etc/apt/sources.list.d - 
Update current distribution
 
sudo apt-get update && sudo apt-get dist-upgrade --autoremove -y
If this brought in a new kernel, sudo reboot - otherwise continue
- Update Ubuntu
 
For the following, keep existing config files when prompted.
sudo do-release-upgrade --allow-third-party
Reboot at the end. If you told the upgrade script not to reboot, reboot manually with sudo reboot
- Change all 3rd-party repos
 
This is after the reboot so do-release-upgrade handles ubuntu.sources and third-party.sources.
The repos will have been disabled: They'll have a repo.list.distUpgrade file, but no repo.list file, ditto for repo.sources. Reenable them by copying their .list.distUpgrade file to .list, ditto for .sources.distUpgrade and .sources.
If you want to do this programatically:
find /etc/apt/sources.list.d -name "*.distUpgrade" | while IFS= read -r file; do
  base="${file%.distUpgrade}"
  if [ ! -e "$base" ]; then
    sudo cp "$file" "$base"
  fi
done
After that step, change jammy to noble.
sudo find /etc/apt/sources.list.d -type f \( -name '*.list' -o -name '*.sources' \) -exec sed -i 's/jammy/noble/g' {} \;
Run an apt update and if it fails on some repos because they haven't been updated to noble, revert those to jammy.
Keep monitoring them and bring them to noble when they are ready.
sudo apt-get update
Bring in the updated 3rd party repos
sudo apt-get dist-upgrade --autoremove
Config ansible.cfg:
[defaults]
interpreter_python = /usr/bin/python3
Playbook noble.yml:
---
- name: Upgrade to Ubuntu Noble Numbat
  hosts: all
  serial: 1
  gather_facts: false
  roles:
    - base/upgrade_noble
Role base/upgrade_noble/tasks/main.yml:
---
- name: Get distribution version
  setup:
    filter: ansible_distribution*
- name: Skip if not Ubuntu 22.04
  meta: end_host
  when: ansible_distribution != 'Ubuntu' or ansible_distribution_version != '22.04'
- name: apt clean
  apt:
    clean: yes
  become: yes
- name: Get filesystem facts
  setup:
    filter: ansible_mounts
- name: Fail if free space on / is below 5 GiB
  ansible.builtin.assert:
    that:
      - item.size_available > (5 * 1024 * 1024 * 1024)
    fail_msg: "Free disk space on {{ item.mount }} is below 5 GiB"
  loop: "{{ ansible_mounts }}"
  when: item.mount == "/"
- name: All apt packages up to date
  apt:
    upgrade: dist
    update_cache: yes
  become: yes
- name: apt autoremove
  apt:
    autoremove: yes
  become: yes
- name: apt clean
  apt:
    clean: yes
  become: yes
- name: Check if reboot required
  ansible.builtin.stat:
    path: /run/reboot-required
  register: reboot_required_file
- name: Reboot if required
  ansible.builtin.reboot:
    msg: "Reboot initiated by Ansible"
    connect_timeout: 5
    reboot_timeout: 600
    pre_reboot_delay: 0
    post_reboot_delay: 60
    test_command: whoami
  when: reboot_required_file.stat.exists
  become: true
- name: Use do-release-upgrade to move to noble
  ansible.builtin.shell:
    cmd: do-release-upgrade -f DistUpgradeViewNonInteractive --allow-third-party
  become: yes
- name: Get distribution version
  setup:
    filter: ansible_distribution*
- name: Fail if not Ubuntu noble
  assert:
    that:
      - ansible_distribution_version == '24.04'
    fail_msg: "Upgrade to Ubuntu Noble Numbat failed"
- name: Reboot on noble
  ansible.builtin.reboot:
    msg: "Reboot initiated by Ansible"
    connect_timeout: 5
    reboot_timeout: 600
    pre_reboot_delay: 0
    post_reboot_delay: 60
    test_command: whoami
  become: yes
- name: Find all .distUpgrade files
  ansible.builtin.find:
    paths: /etc/apt/sources.list.d
    patterns: "*.distUpgrade"
  register: distupgrade_files
- name: Ensure corresponding file exists
  ansible.builtin.command: test -f "{{ item.path | regex_replace('.distUpgrade$', '') }}"
  ignore_errors: true
  register: check_file
  with_items: "{{ distupgrade_files.files }}"
  changed_when: false
- name: Copy .distUpgrade to the corresponding file if it does not exist
  ansible.builtin.command: cp "{{ item.item.path }}" "{{ item.item.path | regex_replace('.distUpgrade$', '') }}"
  when: item.rc != 0
  with_items: "{{ check_file.results }}"
  become: yes
- name: Find all repos
  ansible.builtin.find:
    paths: /etc/apt/sources.list.d
    patterns: ['*.list', '*.sources']
    recurse: no
  register: all_repos
- name: Switch repos from jammy to noble
  ansible.builtin.replace:
    path: "{{ item.path }}"
    regexp: 'jammy'
    replace: 'noble'
  loop: "{{ all_repos.files }}"
  loop_control:
    label: "{{ item.path }}"
  become: yes
- name: Bring in updated repos
  apt:
    upgrade: dist
    update_cache: yes
  become: yes
- name: apt autoremove
  apt:
    autoremove: yes
  become: yes
- name: Pause for 5 minutes for staggered upgrades
  pause:
    minutes: 5