To rename a system or data disk, start by going to the “Disks” tab from your the Server area of your account and click on the disk in question.
Then, click on “Change name” that you see in the upper left-hand corner, and chose a name for your disk. Note that you cannot have two disks with the same name.
Once you have indicated the new name, click on “Change” to validate the modification. It will then be necessary to wait a few minutes.