On the first screen of the Activity (for example Watch TV), the top right orange key gives you access to the Input functions of your devices. Pressing it will take you to a screen where you have access to the input select function of each device in the Activity.
Also there is a feature that can allow you to select the correct input each time. From the Main screen of the Activity, select the gear (settings) icon on the top right of the screen and select Switch Input. You will then see the Video Input Device (normally TV) and the Audio Input Device (if applicable) used in the current Activity. When you select the device, you will be given a list of inputs to test, and when you have found the correct one, select it and the next time the Activity is started the input function(s) will be sent automatically.