Hi Karre
There is no "unpost" function in AR. You should re-enter the cash batch, with all of the amounts in the negative, and using almost the same check numbers (put an "A" or some other suffix to make them unique). Then "apply" the negative amounts to the same transactions, which will bring the unapplied balance of each transaction up to its original amount.
After you post, you will be at square one, ready to re-enter the cash batch properly.
Steve