See the information in Share iCloud+ with your family - Apple Support including the following:
Switch from your personal iCloud+ plan to your family plan
When you share iCloud+, any family members using the free 5GB plan are added to your family plan automatically.
If a family member is already paying for their own iCloud+ plan, they can choose to switch to your plan or keep their own and still remain part of the family. When they switch to the shared family plan, they get a refund for the remainder of their personal plan. They can't keep their current plan and use the shared family plan at the same time.
On your iPhone, iPad, or iPod touch
In iOS 16 or iPadOS 16 later:
- Go to Settings > Family.
- Tap Subscriptions.
- Tap iCloud+.
- Tap Use Family Storage.
In iOS 15 or iPadOS 15 or earlier:
- Go to Settings > [your name].
- Tap Family Sharing, then tap iCloud+.
- Tap Use Family Storage.
On your Mac
In macOS Ventura or later:
- Choose Apple menu > System Settings, then click Family.
- Click Subscriptions, then choose iCloud+.
- Click Use Shared Plan.
In macOS Monterey or earlier:
- Choose Apple menu > System Preferences, then click Family Sharing.
- Click iCloud Storage.
- Click Use Family Storage.