Why does my mac not have free space even after I delete files?
On the assumption that you have APFS file system, macOS 10.14 Mojave, and local snapshots -  the system might take some time to purge the space.

The way to know this would be to open the system information and give it several minutes to catalog all the space into buckets.

Second - check for local snapshots

tmutil listlocalsnapshots /


There are command line options to force a purge of APFS snapshots:

tmutil thinlocalsnapshots / $(echo "10 * 1000000000" | bc) 2



How do I know if a time machine backup is running?
How to delete all local TimeMachine snapshots


If you don't have these, I would suggest making sure you have a good backup and restart to recovery mode and run a disk check to be sure the accounting / filesystem are in good shape.

The next thing to do would be to turn off iCloud and let all those files exist only in the cloud. This will prevent a local storage taking space.

Once you install the upgrade, you can turn on iCloud again and only download the files you need locally.