Q: How can i change my app store to us store

What you need know before you change your country or region

When you change your iTunes Store country:

• Update your billing address to one in your new country. When you change your billing address, the primary address for your Apple ID and your other Apple services, such as the App Store and iCloud, automatically updates.
• You might need to update your payment method. For example, only German credit cards can be used to buy content from the German iTunes Store and App Store.
• You won't see the items that you purchased from the previous country’s store in your Purchased section.
• You won’t see the items from your iCloud Music Library that you matched, uploaded, or added from the previous country’s store.

When you make a purchase in the iTunes Store, App Store, iBooks Store, or Mac App Store, the country in your iTunes Store account information must match the country listed on your Apple ID account page.

Find out what to do if you can't change your country or region.

Information above is from this link >  Change your iTunes Store country or region - Apple Support

See the statement below from If you can’t change your iTunes Store country or region - Apple Support

Your payment method's billing info must match the country of the store you're buying from. For example, only German credit cards can be used to buy content from the German iTunes and App stores.

Change to a different payment method, or update the billing info, then try to change countries again.

Are you currently in the US ? If not then you can't use the store, part of the store's terms is that you have to be in a country to use its store

You would have to be inside the borders the U.S in order to use the U.S iTunes store.

Are you in the U.S.?