What should I do if the C drive is up after upgrading Windows 10?

But it doesn't matter, Windows 10 comes with a disk cleanup function that can help you free up a lot of storage space in minutes.
First you need to type the “Disk Cleanup” command in the Cortana search box at the bottom left of the screen and run it.
Subsequently, select the cleanup object as the C drive, then the system will automatically analyze the items that can be cleaned up, including: temporary files, recycle bin files, old log files and cache. These items can be cleaned up after checking.

If you still have enough C drive space after cleaning up the above items, then you should consider removing the Windows 10 upgrade file and the old version of the operating system file.
These files are also displayed in the file cleanup list, which can be cleared after checking. However, it should be noted that after removing the Windows 10 upgrade file and the old version of the system file, you will not be able to roll back from Windows 10 to the previous operating system.