You have to do it on your phone app. I just did it a second ago. Not sure why Roku doesn't allow you to do this on the device it self. It's weird I couldn't find this option on the website either. It would only take me to the Q&A section.
If you don't have the app you can download on Google Play or on Amazon. If you're like me that don't have a Google account you can go to the Amazon website and download it. Highly recommend it because this app can be used as a backup remote control on your phone if you ever lose or break remote. When you don't use the app you can always "Force to Stop". You have to look through your phone settings for that. It's always good to turn off an app if you don't use it much.