Yes, you can. Designers will work on all the final revisions that you ask for during the handover stage as long as they are in tune with the selected design. But if there are going to be any major changes, then the designer might ask for a reasonable price.
Note that designers aren’t obligated to make many changes to their winning designs, so if you want to ensure your changes will be made, ask for them before you declare the contest’s winner.
The best bet is to talk to the contest holders directly for the changes using the “Activity” tab in the Design Handover stage.
Note that designers are very cooperating to make minimal changes to their winning designs.