Caution On systems with more than one EFI partition, such as systems set up for RAID with mdadm, grub-efi only upgrades one of the EFI partitions, the one mounted to /boot/efi. This can cause failure on reboot, with the system going into BIOS or EFI shell. Discussion here: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1135137
See bottom of this gist for a way to verify EFI before rebooting, and upgrading the second copy so a reboot is safe
To start, read the official release notes.
If your install fits into "vanilla Debian plus maybe a handful of 3rd-party repos", then this guide for a simple upgrade to Debian 13 "trixie" from Debian 12 "bookworm" can be helpful. 3rd-party repos are handled with a find command.