Should I delete a folder called Windows.old ??

Acer laptop, i3 CPU, M 370, 2.40GHz, 2.40GHz, 8GB RAM, 320GB HDD

Win10 x 64, Home, 1903, 24-06-2019, 18362.356

Another looong update 10-09-2019 or thereabouts

284GB occupied, 94GB free on HDD

Regular cCleaner and Auslogics defrag keep it all running as OK as can be expected. 7 years old.

Would like to clear out HDD, who doesn't? The likeliest folder looks like Windows.old @ 9.81GB but trying to rename it to xxxx.yy-DELETE-ME reports 'in use try later'. Is it really in use?

Should one just let sleeping folders lie? As long as they are not known data, eg., Lightroom Catalogue, MS Office work, etc.

See these links:

Delete Windows.old Folder in Windows 10
Free Up Drive Space in Windows 10

These software can be used to see sizes:
TreeSize Free - Quickly Scan Directory Sizes and Find Space Hogs
Antibody Software - WizTree finds the files and folders using the most disk space on your hard drive
WinDirStat - Windows Directory Statistics

Thanks Z. My personal whole disk space reporter is SCANNER by Steffen Gerlach 2009, V2.10. It is free, or was, a nice display, but not fast to get it all logged.

As to how to delete/rename Windows.old or other folders I wish I had old X-TREE where all attributes were on show and changeable. To find a locked file I will have to search & test 220,000 files in Windows.old. I'd rather watch Brexit in Parliament.

Just leave it, it is in use. The contents will be auto-deleted after roughly 10 days from a version update, usually a bit longer than that, tends to be 14 days on mine.
It is basically your old version of Windows 10 files which enables you to return to the old version easily.

You can manually empty that folder before then by going to:
Settings > System > Storage > Temporary files > Tick the box for Windows update Clean-up or whatever.

You can also use Storage sense or the old Control Panel route if you wish.

No need for all those complex options in the tutorials.

2165 and Helmut. Thank you for replies.