Cancel 50GB plan

That is entirely up to you. Your iPhone's local capacity and iCloud storage have nothing to do with each other. They are separate storage locations for different things. iCloud Storage is for backups, iCloud photos, contacts, calendar, notes and an y files and content you may have in iCloud Drive.

Check your iCloud storage at Settings->Account(top of settings)->iCloud->Manage Storage to see how much iCloud storage you are using. And wether you can can downgrade to the free 5GB option without losing anything.

Contact iTunes Support for assistance. See the Apple Support articles below for contact information.   

iTunes Support – world wide   -

Apple Support schedule phone call     -

Billings and Subscriptions   -

Apple on Twitter

Follow the steps here: link-> Change, add or remove Apple ID payment methods – Apple Support (UK)

Why can’t you add the new card? what happens when you try?

swati40 wrote:

… Can you do something to fix this?

No, this is a user to user forum. You are not addressing Apple Support here. As users of apple products much like yourself, we have no access to your account, nor can we fix anything directly. We can only offer suggestions for you to try and fix it yourself.

swati40 wrote:

Hello

I tried this steps many times but ma last bill of 0:99 is unpaid and my old card is not working now so I can not enter my new card number. What to do ?

You could try redeeming an Apple Gift card to cover the cost of the outstanding iCloud storage balance. You might be able to add the new card after that balance is paid. The worst case scenario would be that you have a balance in your Apple ID.

FWIW, I pay for my Family iCloud storage with Apple Gift cards. I haven’t had to use my debit card yet in almost 3-1/2 years of using Family Sharing.

You need to add the new card first. Once its been verified and can be charged, you can remove the old one.