For some reason, if you want to change the default resolution of YouTube’s video playback, you cannot do that on the official app. The is no option to do that. There is, however, a modified version of the YouTube app that lets you control the default playback quality of the videos. Here’s how to change YouTube default video quality on Android.
Change YouTube Default Video Quality on Android
For this tutorial, you will need two 3rd party apps – YouTube Vanced and Micro G. The process id simple and you do not even need to root your device.
Guite to Manually Change YouTube Default Video Quality to HD
Step 1: Download YouTube Vanced and Micro G
Although YouTube Vanced is the primary app, it still needs and relied on Micro G for some features including storing your resolution settings. Before you download and install the two apps, you will need to enable Unknown Sources or Install Unknown apps from Settings > Security. After that’s done, go ahead and install both the apps.
If you cannot find the downloaded apps’ apks, by default they should be inside the Download folder.
Get YouTube Vanced and Micro G.
Step 2: Install YouTube Vanced
If you tried to install YouTube Vanced app, you will notice that it doesn’t open the install screen like what you saw when installing Micro G. This is because the APK is split and you will need a special installer called SAI or Split APKs Installer.
Get SAI from the Google Play Store.
After installing the SAI app, open it and tap on Install APKs. Navigate to the Download folder and choose YouTube Vanced. Wait for a few seconds and tap on install. Open the app after the installation is completed.
Step 3: Log in
Although you do not need to log in to your Google account to video the YouTube videos, it’s better if you do. However, do not use your primary account as there is a chance that Google will block it for violating the Terms of Service.
Make a dummy account and sign in. Signing in to an account will make the user experience a lot better.
Step 4: Change Default Resolution
Inside YouTube Vanced, tap on the profile picture located at the upper right corner. Tap on Settings and then select Vanced Settings. Now, choose Video Settings and then select the Preferred Video Quality Mobile and Preferred Video Quality WiFi. The first one is for changing the default quality when on cellular and the later one if for when you’re connected to a WiFi. 
You can also force the videos to play at resolution higher that you screen. This will still reduce the quality to your screen’s resolution but the image will be a lot sharper. But note that playing a video on higher quality will consume more data, so only use it if you have an unlimited data plan or if you’re on WiFi.
