As the needed parts of the upgraded stock firmware are expected to be included in /e/OS-T builds,
I understand, you simply have to download, reboot to recovery, and apply… to upgrade from “s” to “t”.
Just DON’T WIPE OR FORMAT ANYTHING (don’t apply “factory-reset”)
as explained at step (8) or (9) of the general guide i shared [HOWTO] an Unified Install Guide Project for /e/ - #4 by piero