Replacing Bike Stem

I need to replace my stem with a shorter one because right now, I reach forward too much and hunch my back when I ride. I was fitted for the bike last year when I bought it and discussed this with the seller. He said it was okay, but after a year of riding, I find myself still straining my back and often get neck aches after long rides. I also feel like I can't get in a very aerodynamic position because I have to reach so much. (I can post a picture from a race where I think I look pretty hunched over in my form- if anyone would like to critique that would be great). Anyways- has anyone had any experience buying and then replacing a stem? Prices? Bring it to a shop? Thanks.