Change setting on nikon d3100 to record in 1080p?

How can i do that? It's recording in 720p.

Added (1). It used to export in 1080. I must have accidentally changed a setting but i can't find which one.

If it can do it, it'll be in the manual. Check your settings on the menu as well.

Your manual tells you how on page 47.
In the menu, go to the movie settings, right, select Quality, right, select 1080.