How to Correct Credit Memo?

Best way is to reverse the Credit Memo by creating a linked Invoice.
Do this easily by
creating Invoice,
enter Cust no.,
then use Functions>copy document.
Enter Posted Credit Memo number and tick 'Include Header' box
Click OK to bring details into Invoice.
This creates an invoice with exacy details that Credit Memo had, thus cancelling it out completely, allowing user to then enter a correct Credit Memo if required.