How to initiate iOS software updates remotely on managed devices?
Forcing iOS Updates to devices using Profile Manager is only possible if the devices are in the "supervised" state and the iOS version is out of date to begin with.

Device supervision (the highest level of device management, enables MUCH MORE control over devices) is only possible by: 


Enrolling in Apple's DEP program (really only practical for device administrators that order a lot of phones for the company)
Configuring the device through Apple's free OS X App: Configurator 2


Conf2 will enroll the device in MDM and enable "supervision" on the devices. 

http://krypted.com/iphone/enable-device-supervision-on-ios-devices-using-apple-configurator-2/