Asking job for shift change

Hi everyone, so I've been working at my new job (retail) for almost a month now and it's been going alright. I don't really get many hours (like 4 hours and sometimes 8) and they're always changing my schedule (which I understand, it's retail). But anyways, Im scheduled for a shift next Wednesday (I'm not working until Wednesday, since I only got 4hrs this week :/) but I already planned a trip of 3days from Sunday to Wednesday. And I was wondering how I can go about asking my manager if I can switch that day for Thursday? Or should I just ask for it off? this is my first job so I'm kind of scared about asking for a day off so soon. 

usa jobs
usa jobs resume
usa hotel jobs
usajobs
usa jobs federal government
usa job in ksa
usa jobs
usa jobs login
usa jobs gov
usajobs.gov
www.usajobs.gov
usajobs.com
usajobs
usajobs.gov official