Open the Settings app
Tap your name at the top
Tap iCloud
Tap Manage Account Storage or iCloud Storage
Tap Change Storage Plan or Buy More Storage
Choose a storage plan
Tap Buy
Confirm with your Apple ID password, Face ID, or Touch ID
If needed, free up space by deleting unused apps, photos, videos, or files
If needed, back up and move files to iCloud, a computer, or another storage service