Open the Settings app
Tap your name at the top
Tap iCloud
Tap Manage Account Storage or iCloud+
Tap Buy More Storage or Change Storage Plan
Select a storage plan
Tap Buy
Confirm the purchase with your Apple ID, Face ID, or Touch ID
Open the App Store app
Tap your profile icon
Tap Subscriptions
Tap iCloud+
Select a storage plan
Tap Subscribe
Confirm the purchase
Use the same Apple ID on your iPhone
Make sure your payment method is up to date
Check that your device is connected to the internet
Restart your iPhone if the purchase does not appear immediately