I am doing my UG in Aero eng. But I would like to pursue my grad in Mech eng. While writing SOP for the univs,how should I substantiate it

I want to do grad prog in foreign univ in Mech eng. The main reason for the change over s to hav better exposure of all fields than a specified field. While writing the SOP wat should I cite as reasons for the change over. I think i need some strong reasons so dat it would help me get the admission