I want to buy 200 gb. How do I do it?

Click here and follow the instructions. The iCloud+ service is a rental, not a purchase, and will be billed monthly.

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iCloud Storage is a rental service, you will be charged every month.

Note: iCloud Storage does not increase what you have on your iPad.

If you need more space, you need to delete stuff you don't need or save stuff onto your computer before delete.

Read here => iCloud+ plans and pricing - Apple Support