While @Chalupa_Batman 's suggestion can change your address on PM's system, it WON'T help to change the tax
Tax charged on your acocunt is based on the phone number you have and NOT based on your address on the file. So, if you are keeping your Ontario phone number, PM will keep charging you Ontario rate. You have two choices if you want to take advantage of the lower AB tax rate:
To do so you'd have to change your mailing address in your profile.
Log into your account the go to Profile, then you'll see a pencil beside your name? Click on that and you can change it in the next screen and save. Consider it done. However it may take up to 72 hours or so to update. So it may miss your next billing cycle. Hope this helps.