Meet with your event coach and ask for one. It is like negotiating a contract. Have an idea of how much you want beforehand...I.E. books or books and tuition. It is highly unlikely that you would be granted a full ride as a returner so be realistic.
Also know that just because an underclassmen is slower than you does not mean a coach will think you deserve more than them. Underclassmen have more time left to contribute or they might fill a weak event area for the program.
Also beating teammates on scholarship does not necessarily make you deserving either. Your teammate may be on more than he warrants but the coaches sometimes have a hard time cutting scholarships.
If your event coach thinks you are deserving, and there is equivilancy left, he will bring it to the head coach.
Understand it is not a fair system, but you have a right to try to get what you feel you deserve. Most importantly... be tactful and diplomatic when asking and obviously do it in private.