How-to uninstall an Existing Instance of SQL Server
To begin the uninstall process, go to Control Panel and then Programs and Features.
Right click Microsoft SQL Server 2012 and select Uninstall. Then click Remove. This starts the SQL Server Installation Wizard.
Setup Support Rules runs to verify your computer configuration. To continue, click Next.
On the Select Instance page, use the drop-down box to specify an instance of SQL Server to remove, or specify the option to remove only the SQL Server shared features and management tools. To continue, click Next.
On the Select Features page, specify the features to remove from the specified instance of SQL Server.
Removal rules runs to verify that the operation can complete successfully.
On the Ready to Remove page, review the list of components and features that will be uninstalled. Click Remove to begin uninstalling
Immediately after you uninstall the last SQL Server 2012 instance, the other programs associated with SQL Server 2012 will still be visible in the list of programs in Programs and Features. However, if you close Programs and Features, the next time you open Programs and Features, it will refresh the list of programs, to show only the ones that are actually still installed.
For additional details please refer to Microsoft TechNet article