In case you upgraded from Windows 7/8.1 to Windows 10 then there are extra files on your hard disk only wasting your hard disk space. Look for Disk Cleanup utility to delete files unneeded.
If you are sure you are NOT going back and are NEVER going to downgrade to Windows 7/8.1 then it's possible to save even more. Windows 7/8.1 recovery files are there on hard disk waiting to be deleted. Before doing anything, have some coffee and think about it. There is no coming back! Remember you delete your files at your own risk!
My Acer W4-820 only has 32 GB eMMC memory (i.e. my hard disk is 32 GB). I was able to free 7 GB of memory for other apps or files. Acer hard disk use came down from approx 25 GB to 18 GB. That's not too bad. On the other hand I can't downgrade to Windows 8.1 anymore. However, that doesn't matter. I'm pleased with Windows 10 on Acer W4-820.