Extend Cluster shared volume (CSV) Hyper-v

The first step is to extend the lun on your storage device.
Then, look into your failover cluster manager to find the owner of the CSV.
This can be found in the Storage -> Disks section under Current Owner.

Log into the CSV owner node and type in a elevated command prompt:
1. Type Diskpart
2. Type Rescan
3. Type List volume
4. Type Select Volume (wich you have extended on the storagebox)
5. Type Extend

Voila