.bak files are database backups we can restore the Database backup using SQL Server Management Studio.
  (a) Open SQL Server Management Studio in Object Explorer Right Click on ‘Databases’ Node and select ‘Restore Database’

(b)  ’Restore Database’ Dialog will be displayed on the General page

1. The name of the restoring database appears in the To database list box. To create a new database, enter its name in the list box.
2. Select ‘From device’
3. Click button to display ‘Specify Backup’ Dialog
4. Click ‘Add’  to browse the .bak file from the directory and click OK