Scenario: You have accidentally installed SQL Server 2008 R2 Workgroup and you need SQL Server 2008 R2 Standard.  Obviously, you don't want to rebuild your server from the ground up and have extended downtime, so an upgrade option would be sweet.  Luckily, Microsoft has come to the rescue with an easy way to update your msSQL server/instances.

Solution: Go to the Microsoft Licensing center or grab your SQL Server disk/installation media of the correct version.  I.e. if I was running standard, I would grab the enterprise disk to upgrade.  Once you have the disk, follow these steps:

1. Find your installation media and double click on Setup.exe
2. When the SQL Server Installation Center window comes up, click on Maintenance
3. Click on Edition Upgrade
4. On the SQL Server 2008 R2 Setup - Setup Support Rules window, click OK
5. Click Next > on the Upgrade the Edition for SQL Server 2008 R2 window
6. Click Next > on the Enter a product key: page.
7. Click I accept the license terms. and then click Next >
   
8. Select the instance of the SQL Server you wish to upgrade and then click Next >
9. Click Next >
10. Click Upgrade

To verify your SQL Server upgraded, follow this guide here to pull your msSQL version: http://jackstromberg.com/2013/01/how-do-i-find-out-if-my-sql-server-is-32-bit-or-64-bit/