To add or edit Giving Levels, or, pre-set donation amounts, on your campaign wall, you can scroll down and see the Giving Levels Header. Click on the Pen button to edit your Giving Levels details.
To edit a giving level, you can click on the amount of the giving level you would wish to change, and if ever you wish to delete it, can just click on the trash button on the bottom of the screen. Click "Save" to save your changes.
You can also edit the levels by going to your Edit Page tab.