sybok
Fresh install guarantees that you get antiX 26 as intended and it is the recommended way.
If you have a separate /home partition, you could preserve your home during reinstall and align /home configuration manually (by merging with the one stored in ‘/etc/skel/’).
If you do not, you may consider making a backup of your configuration (if any custom changes made), browser settings (export bookmarks and others; I guess that if the browser versions are the same, backing up the entire folder such as ‘~/.mozilla/’ might work… unless it not being the case).
Upgrade requires manually modifying files in ‘/etc/apt/sources.list.d/’ to point to Trixie instead of Bookworm, then run the updater and post-install changes (such as aligning configuration in your home directory, possibly swapping some packages related to your default init).
See also Upgrade do and dont’s topic on this forum.