System Data taking up to much space to update OS.

Some things to check...

Look for iOS backups…

/Users/YourUserName/Library/Application Support/MobileSync/Backup

Purging local backups

Please note that although this doesn't affect your remote backup from Time Machine, this will get rid of the redundancy (at least until the next Time Machine backup) that a local backup disk will provide. If you need such redundancy or are worried about the recovery of your data then you would be best served to let macOS determine when to purge these files.

Start Terminal from spotlight.

At the terminal type tmutil listlocalsnapshotdates 

Hit enter.

Here, you'll now see a list of all of the locally stored Time Machine backup snapshots stored on your disk.

Next you can remove the snapshots based on their date. I prefer to delete them one at at time. Once my "System" disk usage is at an acceptable level, I stop deleting but you can delete all of them if you want to reclaim all of the disk space.

Back at the terminal, type tmutil deletelocalsnapshots YYYY-MM-DD-HHMMSS , where will be one of the dates from your backup. This will be in the form of xxx-yy-zz-abcdef. Try to start with the oldest snapshot.

Hit enter.

Repeat for as many snapshot dates as required

http://www.thagomizer.com/blog/2018/03/27/cleaning-up-time-machine-local-snapshots.html

tmutil deletelocalsnapshots / # deletes all the snapshots

Thanks to Bob Harris file sizes, Applications -> Utilities -> Terminal:...

sudo du -hx | sort -h 

sudo will prompt for admin password but not reflect what you type, so hit enter after carefully tying the PW. :)

Do you use Time Machine on your MBA on a regular basis. If yes then launch Disk Utility and look for local snapshots:

I would leave the latest local snapshot and delete the rest.

That computer comes with 256 GB of storage.