It is not possible to downgrade Bandwidth resource for a VPS NG subscription on Upgrade Server screen in CCP. The dropdown/slider shows bigger values only, and once the bandwidth is increased, the lower values are not shown anymore.
The issue is acknowledged as CCU-14668: Bandwidth downgrade doesn't work
The issue is permanently fixed in OSA 7.0.
Contact TAM/PTA in order to check the possibility of a fix for 6.0 version