A ".dwg" file extension indicates that the file is an AutoCAD Drawing file. The ".bak" file extension indicates the file is a backup file. When you back up a ".dwg" file in AutoCAD, the program applies the ".bak" file extension, but does not make any additional changes to the file itself. To change a ".bak" file to a ".dwg" file, rename the file extension in Windows Explorer.
Right-click the Windows "Start" button and select "Open Windows Explorer."
Navigate to the ".bak" file on the hard drive or storage device.
Right-click on the ".bak" file and select "Rename" from the context menu to highlight the file.
Type ".dwg" in place of the ".bak" file extension. If the file extension is not visible, type the entire file name in quotes including the ".dwg" extension. For example, type "filename.dwg" -- including the quotes -- replacing "filename" with the actual name of the file. Typing the entire file name and extension in quotes forces Windows to rename the file even if the extension is hidden.
Open the ".dwg" file with AutoCAD or another compatible program.