It's better to create a new model and not mess with the stock models (competition and standard). Here's some notes on editing:

If you want to do editing with testing, the following procedure is recommended:

1. Save Model as ... and give it a name and description you like.
2. Edit some values, click "Test"
if you like the changes, save the model
if you don't, reload the last saved model or remember which parameter you changed and try other values.

After editing some parts/parameters (without clicking "Test") clicking "revert part" brings back the parameter values as they were ON OPENING THE EDITOR. The button is meant to be used if a part/value got messed up while editing. Especially in engine or Aerodynamic interactive plots. If you "Test" once, you cannot go back.