If you cancel the gig within the cancellation period, there is no problem. In the app, you click on ‘cancel this gig’.
Is this not the case? Then you can arrange a replacement via YoungOnes. Click on your gig: ‘Arrange replacement via YoungOnes.’ With this option, the gig is reopened as a ‘replacement gig’ in the app. This allows other freelancers to respond to it. You will see who responds and you choose someone yourself to take over the gig.