Its quite simple actually. Click on the little scroll with the red ribbon labeled Quests. Scroll down (if needed) and click on the Quest called Promotion. If you are not already above the rank, or are the rank Knight, then it should have a smaller box somewhere else on the Quest screen that says Knight. Click on the word Knight and it should have a rather large box located to the right of the list in which you clicked on Promotion. It should have a detailed description of how to obtain Knight, along with its benefits.