How do I switch back to the Japanese store on my iPhone?

I downloaded a free app from the US store last week. I live in Japan and my credit card is Japanese so when I try to update my apps (PicFrame) I get the error message that (Your account is not valid for the U.S. store. You must switch to the Japanese store.)

How do I switch stores on my iPhone? Or, must I switch stores on my iTunes on my computer?

I think I have already switched stores on my computer but it hasn't yet changed on my iPhone.

Any help would be appreciated.