Find your Forms by going to Contacts & Activities > Forms. Here you can make edits to the forms you created, or create new forms.
When you are creating your form, you'll notice that there are two options near the top that say "Edit Confirmation Message" and "Edit Registration Closed Message."
The "Confirmation Message" will be the text that appears when someone has submitted their form.
The "Registration Closed Message" will appear if someone visits this form after registration has closed.
You can always go back to editing the form itself by clicking "Edit Form."