How do I back up my Macbook Pro in iCloud?

As Both previous posting have mentioned.

As good computer practices, the user should protect their important personal information and files, unless they are not important enough to protect all ?

 Do you have a Rescue Plan In-Place. This comprises having 3 Backups using 2 methods and 1 Off Site incase of Theft, Loss or Natural Disasters. A Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS. 

For addition purposes - two Tested Bootable Clones each to separate external drives. This is insurance incase the upgrades goes sideways. At least one Tested Clone and / or Time Machine Backup should be Off - Site

You don't. iCloud can be used to backup iPhones and iPads, it can not be used as a backup destination for a Mac.

iCloud isn't meant for backup, it's meant for syncing data.

pCloud, Dropbox, & other services are meant for backup.