how to change from american app store to canadian app store on i-phone 4s

Hi,

Somhow I managed to get my app store on my iphone 4s from a canadian app store to american app store I was wondering how to switch it back so I can update my downloads from the canadian app store. I have tried to fixed but unsuccessful after trying to us a qr code fro an american app it took me to an american app store but won't let me back to the canadian to download my updated content.