These instructions can be used to upgrade Raspberry Pi OS from Buster to Bullseye with only an SSH connection (no physical access). Some addidional steps are performed to make sure that the Pi automatically joins the WiFi network after upgrading. This is necessary if the Pi is located in a remote or hard to reach location.
Especially on older Pis, the upgrade will take several hours, so plan accordingly.
If you have physical access to the Pi, consider making a backup of the SD card with the SD Card Copier utility (red berry menu in the top left >> Accessories >> SD Card Copier).
If the SD Card Copier is not in the menu, you can install it with sudo apt install piclone.