If you have reclaimed tax against a donation and later need to reverse the claim against that donation you should open the Gift Aid claims area under Financial > Gift Aid > Previous Claims
Find the donation/s in your previous claim and the constituent's donation.
Click on Claimed (image 1 below) and change the status from Claimed to Reverse (Image 2 below).
Confirm the change by clicking on the tick box as shown in Image 2.
Image 1: Status is shown as Claimed
Image 2: Status changed to Reverse
When your active claim is refreshed you will need to enter the reason for the adjustment e.g. Constituent no longer eligible.
Confirm the adjustment by clicking Save.
Repeat the steps above for each donation to be reversed.