As a developer, I am writing a commit message to rollback an update. In my message I want to describe what I am doing which is the opposite of an update. I am looking for something like the word downgrade for upgrade.
If you wonder what is the difference between update and upgrade, check this link. What’s The Difference Between Update And Upgrade?.
The word downdate quite exist (at least on the Urban Dictionary) but it means a bad update.
From your response to my original comment I see that you are looking for a word that describes the move back from the later version c.d to the earlier version a.b.
This means you are reverting to a.b so the action is a reversion.
This is different from the case where you have a third version, e.f, which is an update from c.d which removes features introduced at c.d but has cosmetic differences from a.b This would still be an update from c.d but would be a functional downgrade.