A Contract Request can only be updated if it has been returned to the requester by one of the approvers.
When an update has been made to a returned request, this will enter the workflow again, as if it was a new request.
See the below SharePoint article for detailed information:
https://pitt.sharepoint.com/sites/purchasepaytravel/pantherexpresssystem/SitePages/Updating-a-Returned-Contract-Request.aspx