How Do I Change The Loco In A Scenario?

I'd like to learn how to take an already available scenario and replace one or all of the locomotives with a different model. Can someone help me learn how to do this?

Thanks!

 

Here’s a video that shows you how to change a loco to an existing scenario (dont forget to first clone the original scenario and edit only the cloned version - at least when you are still practising.)

 

Thanks, I'll give it a try when I have some time.