This question gets asked all the time, because the way Workout Trainer manages workouts is pretty messy.  Naturally, there's no help system to answer questions like this, and the way it works differs between the moble app and the web system.

As I recall (and I have to remind myself every single time I do it, because it's far from intuitive), on the app, when you open the workout there's a three-dot menu in the upper right which allows you to delete. I'm on Android; Apple users will probably be slightly different.

I far as I can tell, you can't do this from the app.

You have to use the skimble website from your browser, this is a little easier from an actual computer, but on your mobile device you just have to use your browser to go to skimble.com, log in, find the option at the bottom of the page that says something like "full website" that will take you to the full functionality of the website. You can then find your workouts, click on them, and after that you will see the option to copy, edit, or delete. The mobile website won't have the option to delete. Clicking the "desktop site" option in Chrome will give you a different layout but you still have to find the "full website" option at the bottom of the page to get the delete workout function.