How do I change the network provider for a iPhone 6?

1. Make sure the iPhone is not carrier locked (if locked, then only that carrier can official unlocked and if you qualify)

2. If number 1 is good, just put the nano SIM in and activate.

Read here: If you can't activate your iPhone - Apple Support

Ask the person you got the iPhones from.

Note: Most carrier will only unlock if you're a customer and in good standing.

Well, until you get them unlocked, you got a pretty brick.

Check Settings > General > About > Service Provider > see here.

Also Note: Apple do not unlock iPhone that is lock by a carrier.