You can create a custom form submission message for your forms. To do this, please follow the steps below.
1) From the Forms module, click the form you would like to create a custom form submission message.
2) Select the tab "Notifications and Confirmations". Select "Submission Message". Click "+ Add New Confirmation".
3) A window will appear. Enter a confirmation name, select "Custom" as the confirmation type, add a title, add a message, and click save.
Once complete and a form is submitted by a contact, the contact will see the form submission message with the updated text.