how can i make the app dtore ask for my nee user password. it keeps poping my old user when trying to purchase apps

What to do after [or before] you change your Apple ID email address or password - https://support.apple.com/HT204071 - After you change your Apple ID email address or password, you might need to update your sign-in information with any Apple services you're using.

Go to Settings > Apple ID, iTunes, iCloud and App Store

Tap your Apple ID

Sign out

Sign back in with the correct ID and Password

Apps are permanently connected to the Apple ID used to purchase them.