The accepted answer is incorrect.
The suggested solution changes the text of the "complete" button, not the text on the screen after the survey has completed
Re: How to change the complete survey message "Thank you for completing the survey!"
Answers approved by surveyjs Support
Hello Tim,
There are two properties:
completedHtml and completedHtmlOnCondition.
We use them both in our NPS example.
Thank you,
Andrew
SurveyJS Team