If you find that you have accidentally over-claimed Gift Aid from HMRC, the way to correct this is as follows:
Identify the value that needs to be returned.
On the next Gift Aid Claim that you submit, find the Adjustment field on the Online Claims tab.
Click Edit.
Enter the value that you have over-claimed here.
Click Save.
When the HMRC receive this claim with the Adjustment figure, they will subtract that from the value of the submitted claim, to balance the net total.