How do I delete updates not yet installed?

Remove WU downloads from the temporary folder:-

Stop the Automatic Updates service

Click Start, Choose Run.
In the Run box, type services.msc.
Click OK.
Right-click the Automatic Updates service.
Click Stop.
Stopping the service will take a moment.

Delete the contents of the Download folder
Click Start. Choose Run.
In the Run box, type %windir%\SoftwareDistribution
Click OK.
Open the Download folder.
Delete all contents of the Download folder.
Close the window.

Start the Automatic Updates service
Click Start. Choose Run.
In the Run box, type services.msc.
Click OK.
Right-click the Automatic Updates service.
Click Start.
Starting the service will take a moment.

By deleting the contents of the Download folder - you have removed any downloaded updates.