Once your deal is set to status "won" or "settled", all information will be locked and uneditable.
If you wish to make some adjustments, you should change status of the deal back to open by following those steps:
1. Changing deal status from Settled to Paused (click here for more info):
Once deal status is back to "open", you will be able to make needed adjustments.
Make sure that all the information is correct before you have the deal marked as settled again.