Can I delete my files if they are backed up on iCloud?

iCloud Photos is a syncing service. Any photo you delete from a device where iCloud Photos is turned on will also be deleted from iCloud and from any other device connected to iCloud Photos. You can potentially reduce the storage space required on a device by turning on Optimize Storage. See the following Apple article for more information: Set up and use iCloud Photos - Apple Support

Apple does not currently offer cloud-only storage for photos. If that is what you want or need, you may want to explore 3rd party options such as Google Photos or Dropbox.

Another way that photos may be on iCloud (if you don’t have iCloud Photos turned on) is via a backup of an iOS device. The only thing you can do with a backup is use it to restore a device or set up a new device. The contents of backups are not viewable at iCloud.com and if you delete a photo from a device it will not be included in future backups and will be lost.

If really all your Photos are in iCloud Photos right now (check it at www.icloud.com) , you could temporarily move your Photos Library to an external volume to free storage. But do not delete any items using the Photos.app. Just move your complete Photos Library from the Pictures folder to an external storage. This will not delete the photos from iCloud, just save storage on your system volume.

Move the library back to the Pictures folder, once you have updated the system, before you start Photos again.

Alternately, keep the copy as an additional backup and create a new, empty library in your Pictures folder ( Create additional libraries). Make the new library your system photos library in the Settings > Photos > General (System Photo Library overview), then enable iCloud Photos for it, also Optimize Mac Storage. The library will now be small and fully optimized right from the start.