When Local Disk C is full, your computer may become slow, Windows updates may fail, and some programs may stop working properly.
The first step is to delete temporary files. Press Windows + R, type %temp%, and delete the files inside the folder. If some files refuse to delete, skip them.
The second step is to use Disk Cleanup. Search for Disk Cleanup in the Start Menu, choose Local Disk C, then select Temporary files, Recycle Bin, Thumbnails and Delivery Optimization Files.
The third step is to uninstall apps you do not use. Go to Settings, Apps, Installed Apps, and remove unnecessary programs.
The fourth step is to move large files such as videos, photos, downloads and documents to Local Disk D or an external drive.
The fifth step is to clean the Downloads folder. Many users forget large files there, and this can quickly fill the C drive.
Do not delete files inside the Windows folder unless you know exactly what you are doing. You may damage your operating system.
After cleaning, restart your computer and check the storage again in This PC.