Just forget about changing the Thank You page for now. I think it will dawn on you when the time comes.
I didn't know anything about cgi scripting when I first used the script several years ago for my forms, but through trial-and-error, reading through the script and deducing things from the output that the script produces (like the Thank You page sentence) ... I figured it out.
You will figure it out and know where and when to change the wording of your Thank You page. We have given you exactly how to do it here in this thread. So, later you can go back and reread our posts and it might make more sense to you then, and the light bulb will go off.
You may even want to use other attributes of the script. I use the "ok_url" attribute instead of the default Thank You from the script, but I'm not going to get into that with you here. You need to do some research (as Ed suggested) on your own.
The script now works for you, you can publish your forms, so just take this time now to get the rest of your forms updated and available for your site.
Have a good weekend!