Q: I switched to iTunes Store UK and want to switch back to USA

Go to the bottom of the itunes screen and click the flag

Change the country of your iTunes Store, App Store, iBooks Store, and Mac App Store account - http://support.apple.com/kb/ht6119

I can't change the country of my iTunes Store, App Store, iBooks Store, or Mac App Store account - http://support.apple.com/kb/TS5375

App Store [for iOS devices] Frequently Asked Questions (FAQ) - http://support.apple.com/kb/HT2001 when 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."