Hi - the "update" option should pre-fill the amount already delegated so then you can update it to the new amount you want to delegate. I agree it still could use to be more clear though. I plan to at least add a confirmation message if delegating less to make sure the user really intends to decrease their overall delegation.
Let me know if that would cover your use case or not.
RE: Post Promoter JavaScript Voting Bot - Updates over the past month