First, locate the Document that the Signature Request is on. Click on "Manage" at the top of the Document, and then click "Request Signature".
A pop-up screen will appear. At the top, you will see a list of all the team members with a pending Signature Request on the Document. To the right of the team member's name, you will see the option to "Cancel" the Signature Request. You will be asked to add a comment, and then click "Save".