Previously, resolution settings were controlled through the Stadia App, but applied to all of your devices. With this update, we're introducing the ability to control your resolution directly on each device you use to play. In other words, the Stadia app will no longer control the resolution for all your devices and instead only controls the resolution for that particular device. This change will give you more control over how much data you are using on the different devices.
Changing your resolution on a Chromecast Ultra device
Open the Stadia menu by pressing the Stadia button on your controller
Choose Options > Performance
Choose the maximum resolution you’d like to set for this device. It will take effect the next time you play a game on your device.