After a monthly donation is made, the donor will get an email to setup his Donorbox donor account. The setup process is simply adding a password. His donation email will be his username.
If you didn't setup your account via the setup email please use the "Retrieve My Account" link. This will let you establish a donor account.
You will then have access to your monthly donation plans. Click the edit button to the right of the donation plan you wish to edit (see image below).
To change your donation amount, type in a new amount in the text box under "Monthly Donation Amount" and click the blue "Change" button. That's it!