Once again, you’ll have to tap the three vertical dots button to bring up the menu.
Then, choose quality and you’ll be presented with four options: auto (recommended), higher picture quality, data saver and advanced.
If you need to cut back on your data usage then simply hit data saver to ensure your monthly data allowance doesn’t get eaten away too quickly.
The higher picture quality setting looks like 720p is the maximum resolution it will go to.
While data saver looks to max out at 480p, but can go as low as 144p if offered.
If you want to make sure YouTube always shows video in data saver mode then you set this as the default streaming option in the Settings section of the YouTube app.