There currently isn't a method in the Surveys application for editing the thank you message. However, you can create a standard business process which allows for that functionality. Refer to the information at https://doc.laserfiche.com/laserfiche.documentation/en-us/Default.htm#../Subsystems/ProcessAutomation/Content/Forms-Current/Start-Events.htm#MessageStartEvent and the section that discusses the options on event completion.