$WINDOWS.~BT or $WINDOWS.~WS folder are created usually in the root of Windows installation drive when windows 10 is upgraded. This folder contains a backup of last version of Windows. Such type of folders can also be created if you install Microsoft Office. This folder consume 5 to 6 GB of disk space and cannot be deleted by simply pressing the delete button.
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To delete this folder and recovery valuable storage space. Press Windows Button and Type Disk Cleanup. or press Windows Button + R and type cleanmrg.exe. This will open Disk Cleanup App.
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Select the drive you wish to clean and press OK.
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Once it is done "Clean up system files" button. By Default it will not delete windows system files or update files.
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It will again press and calculate the files and size.
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Now check the folders you want to delete and press OK. This remove the $WINDOWS.~BT or any other update folder and reclaim the disk space.