Unable to download apps in iPhone due to billing info

You must update your payment, method:

1. Open the Settings app.
2. Tap your name.
3. Tap Payment & Shipping. You might be asked to sign in with your Apple ID.
4. If you need to update the billing information of an existing card, tap the payment method, then tap Edit.
5. Enter your new billing information, then tap Done.

See this link for further reference

If you need to change or update your Apple ID payment method - Apple Support