delete existing database and restore the selected bak file in sql

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VB

Windows

SQL-Server-2008R2

Hi all

I made a application where i am taking backup of sql database and restore them.
while restoring first i m deleting the database from sql and then i m browsing for the .bak file and restoring.

now i want is as the database is already present in the server, if i m clicking the restore button after selecting the particular database, it should automatically first take the backup of the existing file at any path and then delete that and last it should attached the new file..

Please need some idea how to so this..

Thank you