You can turn on HD video streaming on your device if you're on a plan with optimized video streaming. 

Activation steps

If you don't have a plan that includes HD streaming, refer to Find the right plan for you​ to add a plan today.

From T Life

1. Access T Life on your device.
2. Select Manage then choose the Gear icon.
   
3. Select Device settings > Video quality.
4. If you have multiple lines on your account, tap the phone number to choose a line.
5. Next to HD Video Resolution, toggle it ON or OFF.

From T-Mobile.com

1. Log in to T-Mobile.com with your T-Mobile ID. If you don't have one, register for a T-Mobile ID​.
2. Select PROFILE. 
3. Go to MEDIA SETTINGS.
4. By HD Video Resolution, set the option to ON or OFF.

HD video resolution details

• Activating HD video resolution only provides the ability to enable higher-resolution video streams by turning off video optimization. It doesn't change the actual, available resolution of streaming video.
• Video resolution isn't determined by T-Mobile, but rather it's determined by the video content provider like YouTube or Netflix.
• Once you turn it on, HD video streaming availability should take effect immediately, but it may require closing and re-opening the app or browser window, or restarting your device.

Full terms

All on-network data used, including free streaming data, counts toward the heavy-user threshold of 50GB in a billing cycle, after which a T-Mobile-branded customer will no longer receive highest priority on the network. When an HD video is active, streaming high-definition video will use data much faster than optimized video, and brings up to the possibility of de-prioritization if you use enough data to reach that limit in a given month. (Learn more about T-Mobile's Open Internet disclosures.)