If your sources file references the release name (e.g.:bookworm), you change them to trixie, then “apt update && apt dist-upgrade”.
or,
If your sources file directly reference distro-suites (e.g.: stable), you just “apt update && apt dist-upgrade” since stable is now pointing to trixie.
In the first reboot, you run “apt autopurge” to remove packages which are not needed anymore.
If your sources file references the release name (e.g.:bookworm), you change them to trixie, then “apt update && apt dist-upgrade”.
or,
If your sources file directly reference distro-suites (e.g.: stable), you just “apt update && apt dist-upgrade” since stable is now pointing to trixie.
In the first reboot, you run “apt autopurge” to remove packages which are not needed anymore.
…and you’re done.