Hi kalisaxo
welcome to the Apple (user-to-user) Community!
Re: " .... issue of a payment declined method "
Guessing you have already checked the expiry date on your card is OK.
Seems, Apple can accept most credit / debit cards used in Rwanda ...
Apple support have an Article online that may provide help with this issue:
If your payment method is declined in the App Store or iTunes Store - Apple Support
Eg: You may need to check that your card-provider is authorized to make international payments.
(seems iCloud storage plans are billed in USD in Rwanda)
or
to ask your credit-card provider why they decline payment from your card
eg: ... there may be changes to local laws ?
else: if you have debated a charge from Apple:
you may need to ask the card-provider to allow payments to Apple again.
________________________________________
Re: "and I want to upgrade my Icloud storage"
The "Related article" above your question lists iCloud plans and pricing.
There's a short video online that may be helpful when upgrading
https://www.youtube.com/watch?v=RcqL6JqyJQQ
All the best :-)