There are two main ways:

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.