How can I uninstall an obsolete SQL Server Instance?
Open Apps & Features.
Choose the Microsoft SQL Server you want remove and click uninstall.
Choose remove in the pop up that opens.
A pop-up will appear. Choose the instance you want to remove in the drop-down menu and click next.
Select the features that should be remove. In case you want to uninstall the Instance completely select all features. Click next.
Accept the removal rules. After your approval the Instance will be removed. Once removed you will be notified.