To Reverse a donation, there is a Reverse Donation Button. I show that button is not currently added to your the Page Layout for your Donation Records. To add the Reverse Donation Button to the Donation Page Layout, do the following: 
  1. Go to the Donation Record. 
  2. Click Edit Layout in the top right corner of the page. 
  3. In the menu at the top, choose Visualforce Pages. 
  4. Look for Reverse Donation Button and drag it to the location on the page layout below (near the Reversed check box is recommended). 
  5. Click Save at the top of the menu. 
 
Now you should be able to click the Reverse button to Reverse the donation.

You can find more information on Reversing a Posted Donation here.