Or the YouTube app? Please be more specific when asking for help, that way we know exactly what you are referring to and are able to pinpoint and try to fix or find a solution as fast as possible.
There are several threads on this same topic already. While a very few were lucky to have 1080P. The majority are stock with 720P. I've not come across a work around that works 100%.
Here's one of the threads where we are discussing a fix for the 1440p and 1080p on the YouTube app. It starts out as the Verizon and Sprint update where we lost the HQ on the app. http://forums.androidcentral.com/showthread.php?t=471405
You can always use the Chrome browser and once you find what you want to watch select the menu on the upper right and select "Request Desktop Site" /It will give you the option for any available resolution including 4K.
Same with the desktop sometimes it looks great in 2k like the sample videos for example however videos where people talk like MKBHD it is like a second out of sync even in 1080p