In order to cancel transaction please find the edit link from the left hand side menu within required transaction. If this option is not available please contact your account administrator and request to enable this permission for your user.
If a transaction is cancelled it will cancel the whole file. It will save all of the information within the transaction and log all activity. After a transaction is cancelled you will be able to upload documents and complete anything you need to.
You can pull the Cancelled sales report if you have access to the Reports section and also change your filter on the transactions page to view Closed & Cancelled transactions.