How can I move a tag on a git branch to a different commit?

Use the -f option to git tag:

-f
--force

    Replace an existing tag with the given name (instead of failing)

You probably want to use -f in conjunction with -a to force-create an annotated tag instead of a non-annotated one.

Example

1. Delete the tag on any remote before you pushgit push origin :refs/tags/<tagname>
2. Replace the tag to reference the most recent commitgit tag -fa <tagname>
3. Push the tag to the remote origingit push origin master --tags