
Resolve Windows 10 upgrade errors contains a link to a tool that will help you diagnose the failure contained within. The folder contains a log file, if the upgrade to 1903 is indeed failing, the log would contain the reason. The second is that your system has been trying to install 1903 but failing to do so. There two possibilities, the first is that this folder isn't all that new, and contains the temporary files from the upgrade to 1809. The folder is used as a temporary storage location when a feature upgrade is installed, but there is supposed to be an option in the Disk Cleanup utility that will delete the folder. Which is the reason, the option to clean up system files, isn't removing the directory. The files contained with $WINDOWS.~BT are not technically system files. Use the button Clean up system files to compute how to remove the $WINDOWS.~BT folder. Attempt to launch cleanmgr on the C:\ drive.
