You can edit the gift from the Gifts Grid in the Fundraising Module or from the Gifts Tab in the constituent's record by clicking on the amount. You can also select the row of the gift in the Gifts Grid, and use the View/Edit pull down menu and select Gifts.

The screen for the gift will open and you can make changes to any field except the gift type, which cannot be edited. You can cancel the edit by clicking on the Cancel button on the top right. Once the changes are made, click on one of the green Save buttons at the top or bottom of the screen to finalize the edit.