Follow the steps below:
1. open cmd as admin
2. vssadmin list shadowstorage (to check the usage)
3. vssadmin list shadows (to check the restore points)
4. vssadmin resize shadowstorage /on=<drive letter>: /For=<drive letter>: /Maxsize=4GB
5. check storage size now (via Explorer) - it should be free
Reference: https://indrajitc.wordpress.com/2008/03/25/reclaiming-disk-space-from-system-volume-information/