iCloud & iCloud Photos · iCloud storage space and plans
How do I upgrade or downgrade my iCloud+ storage plan on a Mac?
Short answer: In System Settings → your name → iCloud → Manage, choose Change Storage Plan to upgrade or Downgrade Options to move to a smaller plan. Billing goes through your Apple Account. Check your data fits before downgrading.
Open System Settings, click your name, choose iCloud, then Manage and Change Storage Plan (wording varies slightly between macOS versions). Pick a bigger plan to upgrade, or choose Downgrade Options to move to a smaller or free plan. Apple's prices are shown there and charged to your Apple Account.
Before downgrading, check that what you store fits the smaller plan — see what happens if I downgrade below what I use. Storage Studio doesn't sell or change iCloud plans; see Storage Studio and your iCloud plan.