1.
Sign in to your Apple ID account page and edit / update your Apple ID details.
https://appleid.apple.com
On the Sidebar click Payment methods -> then tap Payments Methods and tap Manage payment method.
Or -> Change, add or remove Apple ID payment methods
Change, add or remove Apple ID payment methods – Apple Support (UK)
See also -> Payment methods you can use with your Apple ID
Payment methods you can use with your Apple ID – Apple Support (UK)
You can also use your Apple ID balance as payment method:
Add money to your Apple ID balance
Add money to your Apple ID balance – Apple Support (UK)
Check your Apple ID balance
Check your Apple ID balance – Apple Support (UK)
2.
Check -> Go to -> Settings -> your name -> iCloud -> if you see 50 GB (or 200 GB or 2 TB) then you have an iCloud subscription.
If you have an iCloud subscription, you will receive a monthly bill from Apple, check your email box.
You pay monthly for iCloud storage.
If you have upgraded iCloud storage, it would take 1 to 2 business days.
Once your request for iCloud+ has been activated, you should receive an invoice in your email box.
See -> How to check the storage on your iPhone, iPad and iPod touch
How to check the storage on your iPhone, iPad and iPod touch – Apple Support (UK)