What computer operating system for your Premiere Elements 12? Have you updated 12 to 12.1 yet using an opened project's Help Menu/Updates?
For your immediate question....
If you mean "How to convert a colored clip to black and white?", then Expert workspace, see fx Effects/Image Effects/Black & White.
If you are referring to something else, please give more details.
Now I've learned how to do
Thanks for the follow up and wonderful news that you have done the conversion to black and white effect successfully. Great job.
Best wishes with your Premiere Elements projects.