For similar words, the antonym is obtained by simply substitute the substring up with down like in the following:
- upgrade - downgrade
- upload - download
What about update? I searched for something like downdate but it has very few occurrences and I'm not sure if it is only a rare used word or a wrong one.
A bit of context
The use is related to computer specific context. If I think of update's meaning as up-to-the-current-date, then the word that I need to use should have the meaning of up-to-the-past-date. As suggested in an answer, good candidates can be rollback, reverting and restoring. However, all of them suggest the idea of modify something in order to make it equal to a past state. In my case it's a bit different. It's a chunk of code that works well with current version of some programming language API but since it needs to be run with an older version I modified it to be compatible with this older version. Maybe in this case I could just say 'updated to the previous API version' but I wonder if the use of downdate is correct.