Every receipt for a monthly gift carries a one click cancel link. You can also cancel from your passport. Cancellation stops the next charge; it does not reverse charges already settled.
Refund requests go to the committee that received the contribution, because the committee holds the funds. Reply to your receipt email and it reaches the committee's treasurer. Committees generally honor refund requests received within 60 days. Electy will help locate any gift by its deposit ID.
Your card statement shows the committee's name so you can recognize the charge. If you do not recognize a charge, contact the committee through your receipt first, then your card issuer.