I'm trying to activate the NSX license from vCenter but it shows 0 CPUs usage. Any suggestion on how to change the NSX license key because also I'm not to remove this license from vCenter.
There is no option to remove the license and enter a new one.
You actually have to assign it as a solution license by selecting the "assets" tab in the licensing section, and choosing "solutions". From there you'll assign your license key as described in the Add and Assign an NSX for vSphere License section of the admin guide.