increasing sequence resolution in premier pro
First variant. You can change "Frame size" in "Sequence Settings" (Sequence -> Sequence Settings) to more higher. You need to change them with saving proportions of your video frame.
To save 720p quality on youtube you should set amount of vertical pixels to 720 or more. With 1080p it should be 1080p or more.


Second variant. You can change resolution of your video file on the render stage.
Move to Basic video settings, then uncheck checkbox for changing resolution. And then you can change it to needed. With height to 720p or more.
Also you can save proportions of frame.