http://support.apple.com/kb/HT2001when using iTunes --> "To change countries, scroll to the end of the iTunes Store home page and click the flag indicating the current country. To choose a different country, click the appropriate flag."

iOS: Changing the signed-in iTunes Store account -http://support.apple.com/kb/HT1311  - 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."

You will, of course, need to change your address and enter valid credit card information after you have changed the settings.

iTunes Store: Changing Account Information - http://support.apple.com/kb/HT1918

Further tips on changing country - https://discussions.apple.com/message/20029737 - problems you encounter when changing iTunes country