how do I back up a windows PC using icloud

Apple offers iCloud Backup for iPhones and iPads but not for Mac or Windows computers.

It is possible to sync some, but not all, data from a Windows PC with iCloud. Syncing has some of the benefits of a backup (and the benefit of being offsite) but is not the same as a backup. Major differences include the fact that not everything on your PC can be synced and the fact that if you delete something, accidentally or otherwise, there is only a limited time period (30 days in most cases) during which it can be recovered.

Also see What is iCloud for Windows? - Apple Support

and Set up iCloud for Windows on your PC - Apple Support

I would encourage you to back up your Windows PC to an external drive or to an online service such as Carbonite rather than iCloud. See: https://support.microsoft.com/en-us/windows/back-up-and-restore-with-windows-backup-87a81f8a-78fa-456e-b521-ac0560e32338

and

https://www.carbonite.com/

Microsoft Windows (from your question) cannot be backed up to iCloud.

iPhone SE (from your footer) can be backed up to iCloud, and can also be backed up to a Windows PC.

What are you trying to back up? An iPhone or iPad? Or a Mac or PC?