Via My CloudVPS it is not yet possible to change the volume type.
Via Horizon, navigate to ‘Compute → Volumes’. Select in the volume menu on the right dropdown ‘Change Volume Type’. In the windows you can change the type.
The volume cannot be attached to an instance. If it is attached, please detach it to activate the type change. The volume cannot have snapshots. If the volume has snapshots, you must delete those first.