You can't change resolution on a Roku TV. Whatever resolution the TV is capable of is what the software is set for. There are no display settings in the TV OS. What is telling you the display resolution?
Dan Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.