This guide was created with Microsoft SQL Server 2008 in mind, but should work for any version of SQL Server.
To remove a mirror you need to be logged in to the Principal server, and authenticated against the database instance. You then:
1. Right-click on the database and go to Properties.
2. Select Mirroring on the left.
3. Select the option to Remove Mirroring.
If you wish to then delete the mirrored copy of the database you can follow the article at Delete a database which has been set up for mirroring.