Designers typically provide minor revisions for free and will be happy to do more detailed revisions for a reasonable price. Minor revisions may include a colour or size change.
Note that designers aren’t obligated to make any 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.
It is expected that the design is complete once it is declared the winner. This way, you and your designer can focus on sharing files in the Design Handover stage.