If this is a cash or check offline gift, follow the below steps: 
  1. Go to Fundraising > Teamraiser
  2. Click Manage next to the teamraiser in question
  3. Click on Event Gifts
  4. Locate the gift in question
  5. Click on View/Edit
If you are unable to locate the gift you may need to search by the donor record:
  1. Navigate to Constituent> Constituent360
  2. Locate the donor 
  3. Go to the Transactions tab 
  4. Locate the incorrect donation
  5. Under the Actions column, Click Edit
  6. Enter the correct gift amount
  7. Record as cash or Record as check
To edit a credit card amount refer to How do I refund a transaction?