Can I buy more space for back up

Read this support article for instructions on how to upgrade to iCloud+ --> Upgrade to iCloud+ - Apple Support

Or if you already subscribe to iCLoud+ go here: How to switch to a different subscription plan - Apple Support

Hi there,

If you need more storage on iCloud, you sure can upgrade to iCloud + and get more storage space.

Follow the instructions here, plans start at $0.99: Upgrade to iCloud+ - Apple Support