How do I change the App Store from US store to Canadian in order to download apps?

You need to use an account for the designated country where you want to purchase the app. You need to make a Canadian account

Change your iTunes Store country

Sign in to the account for the iTunes Store region you'd like to use. Tap Settings > iTunes & App Stores > Apple ID: > View Apple ID > Country/Region.

Follow the onscreen process to change your region, agree to the terms and conditions for the region if necessary, and then change your billing information.

http://support.apple.com/kb/HT1311