Hi, I am Dave, I will help you with this.
For the 24H2 update, you would need at least 40GB of space on the C drive, because the old Windows installation will be copied into a Windows.Old folder, the update donwlaod requires 8GB of space and also some space for temporary files to move data around in..
Click the link below to download a small free utility named Treesize.
https://www.jam-software.com/treesize_free
Very Important - to run TreeSize, you must right click its icon and choose Run as Administrator.
Please provide a screenshot of your C drive in that TreeSize utility, so I can see your hard drive and work out where that space is being used and if there is an underlying problem.
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