You can either do that through Gitea UI

or if you have a lot of repositories to update, the token is in the git config of each repository you have mirrored

Gitea does not keep the token elsewhere in its own database or provide an api call to update the token

here a sample from one of mine (removed any sensititve details)

[core]
repositoryformatversion = 0
filemode = true
bare = true
[remote "origin"]
url = https://oauth2:[here is the token]@github.com/[org]/[repo].git
fetch = +refs/:refs/
mirror = true
fetch = +refs/tags/:refs/tags/