Always refer to the official Release Notes for the specific version one is upgrading to, e.g:
Debian 12 -- Release Notes
Upgrading from one stable release to the next (e.g. bullseye to bookworm) is done by following the release notes for your architecture. For most users, that means the Release Notes for AMD64 or Release Notes for ARM64.
For bookworm to trixie, see the trixie Release Notes.
Content below this point is quite general information/advice and may not be current and/or at all fitting/appropriate for any particular version upgrade.
Back up your data
Performing a release upgrade is never without risk. The upgrade may fail, leaving the system in a non-functioning state. USERS SHOULD BACKUP ALL DATA before attempting a release upgrade. DebianStability contains more information on these steps.
Encrypted Root
On systems which rely on the initramfs integration, one can mark 'cryptsetup-initramfs' as being manually installed so APT never selects it for auto-removal with the following command: apt-mark manual cryptsetup-initramfs.