Q: How can i change my currently store to another store like Saudi's store to USA's store  ?

السلام عليكم ورحمة الله وبركاته

http://support.apple.com/kb/HT1311?viewlocale=en_US

Are you in the US ? You can only use the US store if you have a valid US billing address and you are in the US when buying from it.

App Store 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."

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

You can continue to use items you bought from the other store but you will not be able to upgrade them or download them again.  When you change store countries you lose access to the other store.