When you try to restore a database there are some very tricky stuff you need to know.
When using MSSMS (Microsoft SQL Server Management Studio)
Go to Options and Check the Overwrite existing database
If the database is from another Machine, you ll have to check that the path shown bellow. Go to this path and make sure you have the database mdf file stored in the same location. you can modify that to specify the same path.
Click Ok and you re done.
You might get a message like that. (Exclusive access could not be obtained because the database is in use)
To run this script
ALTER DATABASE yourdatabasename SET MULTI_USER WITH ROLLBACK IMMEDIATE;
Drop database yourdatabasename
Or if you have Profiler, Integration Services Or Analysis Services Make Sure you go to
Computer, Manage, Services And Applications, SQL Server Configuration Mansager, right click and stop extra Services