Uninstalling SQL Server Service Pack or Hotfix

Prior to SQL Server 2008, if you had to rollback a Service pack or hotfix installation, it was a long and hectic process as it involves rebuilding the SQL Server and restoring the user and system databases and setting up jobs, logins,etc... And in the meantime if you have missed backing up something then you are gone!
Starting from SQL Server 2008, Service Pack 1, microsoft has introduced an option to uninstall the service pack or an hotfix. It is as simple as uninstalling an application from your computer.
To uninstall an SQL Server service pack or hotfix,
Go to "Control panel" --> "Uninstall a Program" --> Click on "view installed updates" on left hand side
Choose the update you want to uninstall and click "Uninstall" option.

A failure was detected for a previous installation, patch, or repair during configuration for features...

You receive the following error when you try to apply an update or service pack for a SQL Server Instance.

TITLE: SQL Server Setup failure. ------------------------------ SQL Server Setup has encountered the following error: A failure was detected for a previous installation, patch, or repair during configuration for features [SQL_Engine_Core_Shared,SQL_PowerShell_Engine_CNS,CommonFiles,CommonFiles_Loc,]. In order to apply this patch package (KB2528583), you must resolve any issues with the previous operation that failed. View the summary.txt log to determine why the previous operation failed. Error code 0x84B20001. ------------------------------ BUTTONS: OK ------------------------------

This happens when you try to install an update or service pack or do an edition upgrade and if that action does not complete successfully.
In my case, I recently tried to do an upgrade of SQL Server 2008 R2 Evaluation version to SQL Server 2012 Evaluation edition and since this upgrade path was not supported, the installation gave an exception which forced me to quit the installer.
After this I tried to install SP1 for SQL Server 2008 R2 and I got this error.

To Fix this issue, you have to "Repair" the existing SQL Server Instance. This option is available in "Add Or Remove Programs" feature of Control Panel.

Once the Repair is completed, you can restart the installation of Service Pack.