Q: how to change my identity for iTunes and App Store

Change it how ? If you mean log out of an account so that you can log in with a different account then tap on your id in Settings > iTunes & App Store and then tap 'sign out' on the popup (anything downloaded by your currently logged in account will remain tied to that account ; if you mean change the primary email address on an account : Change your Apple ID - Apple Support (What to do after you change your Apple ID email address or password - Apple Support)