To cancel a reservation you have to have specific permissions on the reservation order. The permissions are not inherited from the subscription, so… contact who created the reservation (or just open a ticket with the Azure Engineers).
Once you have the right account, (or you’ve been given the permissions) go to the reservations tab in the Azure Portal. It’s best to have at least a couple of people who have permissions on the resource. To get to the reservations tab follow the link below:
Select the reservation you want to cancel and click on the name of the resource.
From the top click on “Refund”
Click on return reserved instances once you see the “Refund subtotal” pop up
Please refer to the following docs for additional informations: