User Name and Password

Go to the featured page, and then go all the way to the bottom, and you can click on the account button and choose sign out. Then simply use your account to sign in.

 

If you don't have the iPhone/iPod touch OS 3.0 or later you may have to dig into Settings, and from there to Store in order to sign out. I think also the iTunes account on the device used to be set accordingly to the account to whom the purchased content on the device belongs to. I.e., if you sync the iPod touch with your computer where your purchased iTunes music/videos are authorized, it will input that same account into the Store settings...