How to activate iCloud storage

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If you want to upgrade your iCloud Storage plan, follow these steps:

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1. Open the Settings app.
2. Tap the iCloud Storage Almost Full notification. Or:

• • To upgrade your iCloud+ plan, tap your name, then tap iCloud. Under iCloud+ Features, tap Manage Plan.
  • If you're not already subscribed to iCloud+ and want to buy a plan, tap iCloud, tap Storage (or Almost Full), then tap Get More Storage.

3 . Then choose a plan and follow the onscreen instructions.

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copied from: Buy iCloud+ or upgrade your current storage plan - Apple Support

You can control which apps can use iCloud by going to Settings -> Your Name -> iCloud -> Saved to iCloud -> See All -> switch saving/syncing data to iCloud on or off.

Remember, that iCloud Storage does not extend your local storage space on your iPhone:

What's the difference between iCloud storage and device storage on your iPhone or iPad? - Apple Support