iTunes prompts for billing info.
In most cases, this happens when there is an outstanding amount for the purchases made from the store. Updating our bank account, automatically fixes the issue, as the outstanding amount is deducted. It is also a fact that iTunes, sometimes holds a pre-authorization hold. For example, if you purchase a song for $1.29, an additional amount of $1.29 will be temporarily hold (For some sort of verification, to check if the account has sufficient amount? I honestly don't know), so the for a purchase to go through, there should a minimum of $2.58 in our account. If our account has insufficient amount, then the pre-authorization will be declined and iTunes will periodically try get a hold for the next 7 days, and the prompt to update the billing information or credit credit card will start poping up.
If the above info is not true in your case, then your billing info really needs to be updated.
1. Go to
https://myinfo.apple.com
2. Log In with your Apple ID
3. Update whatever is needed.
4. Check the 'Address book'and 'Phone numbers' - Should match with your credit card.
5. Don't forget to save and log out
And then open iTunes and update the same billing information.