TL;DR:
Account >Personal donations> "Monthly Giving" > select contribution you want to stop > click "Stop Giving"
The Details:
Navigate to your account page and click on "Personal Donations" on the left-hand navigation bar.
Next, you will select the "Monthly donations" tab under Donations.
Here you will see your current (and previous) monthly giving contributions. Click on the active one you wish to cancel. At the bottom of the page, you will see an image of your credit card information, following by a "Stop Giving" hyperlink.
Click on "Stop Giving" and follow the on-screen instructions to cancel your monthly giving contribution.