How do I change the default itunes store account for the app store on my iTouch?

I have a new iTunes account but the default when I go to the app store on my iTouch is the old one, and I don't know how to change it. I can't buy apps directly from my iTouch because of this.|||it should change when you change the account on the itunes store in your ipod touch. other than that i have no clue.