Assuming this is a separate Roku device attached to a TV, not a Roku TV with Roku capabilities built in:
Picture adjustments of this sort are made on the TV, not on the Roku.
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
If this post solves your problem please help others find this answer by clicking "Accept as Solution.".
Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X
