To edit the refund amount, it must match the credit note or payment on account you allocated it to. If it doesn't, delete the refund, update the credit note or payment on account, then recreate the refund.
NOTE:
If you use the Cash Accounting VAT scheme and the refund is on a submitted VAT Return, you can only edit the Method or Reference.
Cause
Resolution
Go to Banking, then Bank accounts.
Open the bank account with the refund.
Locate the refund in the Activity tab.
TIP:
Use the From and To boxes to change your date range. Select More and change the Type to Supplier Refund.