what is the proper way to ask the manager to change the time i start work at least 3 hours later?

okay, i have class registration for college on the 25th of june and i’m affraid i might be working that day. My registering for classes is at 8:40am and my regular hours for work is 8:30am -4:00pm… i want to know how i can properly ask the manager to reschedule that day of work for [...]