word choice - Antonym of the verb "complete"

I have an action on a web form. It's a button whose action is to complete a case.

I need to name another button, and I want to use a verb that conveys the meaning of "undo the completion" of this case.

What's an appropriate antonym for the verb complete?

• uncomplete — is this a real verb?


• decomplete — is this a real verb?


• other?

Button 1 is "Complete Case".

Button 2 needs its opposite.