Click on the Contact form. Note that it may be 'Contact info and form with map selection' as well.
- Go to Appearance and select Customize it manually
- Click on Customize this component.
- At the Customize template section, you can see the 'Thanks for submitting' text circled in purple.
- Go ahead and edit that to a message that you want to customize it to. Let's try 'Thanks for being a valued B12 customer!'