Hi Susanna,
I think this should work, but if not, I’ll need you to share your "share link’ so I can give you more specific advice. You can find more information on how to do this here.
You should be able to go to the form and click on it (either in the designer or in the navigator panel on the left) - Then if you look at the top of the panel on the right, you’ll see four tabs, click on the little “cogwheel” tab. You should see a row in the panel now that says “show state” with 3 tabs beside it (Normal, Success, Error), click through these tabs and you’ll see the different messages displayed on the designer and you can click on them and edit.
Important! - Before you publish the page, make sure you’ve clicked the “show state” tab back to normal or the form will start on whichever “state” it was left on.
Hope that helps!
-Jeff