On the "My Requests" page we have under actions the edit function. However, in the Request form it is missing.
The edit button only appears for requests that are at particular stages of the request lifecycle - the ones we saw without it are in the fulfillment range, where any approvers have already examined the requests and passed it on for fulfillment.
The same goes for a request in the Approval range - the Approve/Reject button does this.
The reason for this is to stop a user editing their own request after it's submitted; only the approver or fulfiller can correct any issues as part of their change.