You should just be able to hit the red 'delete' button on the template that is messed up and it will automatically revert back to the default template.
Oh ok, now I understand. The only way to make that happen would be to do a full forum backup in the Founder Utilities to a date before you had the problem. Note, however, this will effect more then just the templates. It will also bring back deleted posts and members that were deleted after the date you are restoring it to.
For the future, we recommend backing up all your modified templates in a word document before making any changes, that way if you make a mistake while modifying them, you can simple copy and paste what you had back in there.
If you do a full restore, then I believe it should also restore the templates yes. I am not 100% sure as I have never used it to restore templates yet, but from my understanding it should.
You won't lose any posts or members by doing the restore, to be clear, but any posts and members that were deleted between today and the restore date, will return and need to be re deleted.