The short answer is -- if you're logged in as an admin -- head here, and click the three dots next to the donation you'd like to refund / remove.

Here's the longer walkthrough:

Click on your logo in the upper right corner, and select Payments & Payouts:

Next, click on "Payments Received"

Next, click on "Offline"

Finally, click the three dots next to the name of the donor whose donation you'd like to refund / cancel.