To manage existing credits for a member, do this from the History page. Access this by selecting 'History' from the the Appointment Credit contextual menu on their member detail page.
From here you can Add or Remove credits one at a time, also see what and when they used Credits.
In the Credits column the first number is how many Credits have been used/scheduled. Second number is total purchased.
Removing Credits will not refund the purchase amount. Do a partial or full refund on the original purchase receipt.