#remote
The 3900 express comes with an IR remote so there is no pairing it should just work at the box. If you're trying to pair a wifi remote purchased separately, use the IR remote to go in to the settings> remote & devices> pair remote> start pressing and holding pairing button in the battery compartment, release after 30 seconds for a bit and start again until you get a confirmation its paired.
If you don't have a remote to go into the menu, unplug the express from the power, then plug it in and start pressing and holding pairing button, stop when the roku stops bouncing for a bit and start pressing again until you get confirmation its paired.
The 3900 express comes with an IR remote so there is no pairing it should just work at the box. If you're trying to pair a wifi remote purchased separately, use the IR remote to go in to the settings> remote & devices> pair remote> start pressing and holding pairing button in the battery compartment, release after 30 seconds for a bit and start again until you get a confirmation its paired.
If you don't have a remote to go into the menu, unplug the express from the power, then plug it in and start pressing and holding pairing button, stop when the roku stops bouncing for a bit and start pressing again until you get confirmation its paired.