how to open back up file of CAD

You can open BAK files in CAD by first renaming the extension so that it has a DWG extension.
These steps show how to do this using Windows Explorer.
1. Using Windows Explorer, browse to the folder where the drawing (file with DWG
extension) is located. Note: Steps 2 through 8 are only applicable if the drawing is not
missing or misplaced.
2. Highlight the file in Explorer.
3. Right-click on the file name and select Rename from the pop-up menu.
4. Using the arrow keys, move the cursor right after the period in the file name, then delete
the DWG file extension.
5. Enter OLD after the period. The drawing will now have OLD as its file extension. This
saves the drawing in case it is needed later.
6. Press ENTER to complete the change.
7. When the Rename dialog box displays, choose Yes.
8. In Explorer, browse to the drawing file with a BAK extension.
9. Highlight the file in Explorer.
10. Right-click the file name and select Rename from the pop-up menu.
11. Using the arrow keys, move the cursor right after the period in the file name and replace
the BAK file extension with DWG.
12. Press ENTER to complete the change.
13. When the Rename dialog box displays, choose Yes.