How to import a bak file into SQL Server Express

There is a step by step explanation (with pictures) available @ Restore DataBase

1. Click Start, select All Programs, click Microsoft SQL Server 2008 and select SQL Server Management Studio.
   This will bring up the Connect to Server dialog box.
   Ensure that the Server name YourServerName and that Authentication is set to Windows Authentication.
   Click Connect.

2. On the right, right-click Databases and select Restore Database.
   This will bring up the Restore Database window.

3. On the Restore Database screen, select the From Device radio button and click the "..." box.
   This will bring up the Specify Backup screen.

4. On the Specify Backup screen, click Add.
   This will bring up the Locate Backup File.

5. Select the DBBackup folder and chose your BackUp File(s).

6. On the Restore Database screen, under Select the backup sets to restore: place a check in the Restore box, next to your data and in the drop-down next to To database: select DbName.

7. You're done.

To do this via TSQL (ssms query window or sqlcmd.exe) just run:

RESTORE DATABASE MyDatabase FROM DISK='c:\backups\MyDataBase1.bak'

To do it via GUI - open SSMS, right click on Databases and follow the steps below

Using management studio the procedure can be done as follows

1. right click on the Databases container within object explorer
2. from context menu select Restore database
3. Specify To Database as either a new or existing database
4. Specify Source for restore as from device
5. Select Backup media as File
6. Click the Add button and browse to the location of the BAK file

refer

You'll need to specify the WITH REPLACE option to overwrite the existing adventure_second database with a backup taken from a different database.

Click option menu and tick Overwrite the existing database(With replace)

Reference