Hi
It may be possible to select None as your payment method. If you are not able to choose None (even for downloading free apps and updating existing ones), then the following support article explains why that is the case:
Why can’t I select None when I edit my Apple ID payment information? - Apple Support
Follow the instructions here as appropriate:
- Pay an unpaid balance in the iTunes Store - Apple Support
- Change or remove your Apple ID payment information - Apple Support
- Payment methods you can use in the iTunes Store, App Store, and iBooks Store - Apple Support
If you still need help, contact iTunes Support: