First, go to My Requests by clicking View All next to My Requests in Services
Then find the request in question.
If the request is in Drafts, just click Cancel.
If the request is Open, you will need to click Request to Cancel. Since the request is already in progress and approval or fulfillment steps may have begun, the approver or fulfiller will need to complete the cancel action. This is because they may need to undo things they have done.
Request to Cancel will also ask you to fill out a cancellation reason. Just enter your reason in the form and click submit. That reason is provided to the approver/fulfillers. It may help them with undoing their work.