Files with the BAK extension are actually back-up files created by other applications, and can range anywhere from word processing documents to graphic design templates. A BAK file can literally be any type of file. These files are usually created on a periodic basis by programs to prevent total file loss in the event of a program or system crash. Opening files of this type requires a user to identify which application created the file.
Locate the BAK file in your computer's file browser.
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Change the file extension from BAK to the appropriate extension of the program that created the file. For example, if the file is name "mydocument.bak" and you know that it is a world document, you would change the file name to "mydocument.doc" using the DOC extension associated with Microsoft Word.
Double-click the file to open it using the associated application.
Be absolutely certain of which program created the BAK file before changing the file extension. Also, beware of BAK files received as email attachments, as these often contain malicious software.