After logging in to the Member Center, you should see the following screen.
Click the gray "Edit" button in the Contributions section. This will open a popup that allows you to change the amount of your contribution. You can also change the next payment date, which will adjust the date your monthly gift processes moving forward.
Be sure to click the blue "Save" button when you are finished making changes. When you click "Save," a green popup in the upper right of your screen should verify that the update was successful.