How to change the message on limited submission?

Hi, I have a special offer, which I would like to cap to 5 registrants. Upon reaching 5, people can still register, but will see a message which says the Limited Offer is no longer available - normal fees apply. Right now, I can set this with voucher code, but I am unable to change the default message... please advise.

If I understood you correctly you want to change default Thank you message when your form is submitted. 

You can achieve by using conditional logic to Change "Thank You" message.

Here is my demo form: https://form.jotform.com/53341547139961? 

I have used conditional logic to change thank you page.

If Voucher code is equal to 123 users will be redirected to default thank you page and if code is not equal to 123 they will be redirected to customized thank you page.

If I did not understood your question correctly can you please explain in more details what you want to achieve and also provide us ID of the form you are working on.

Hope this will help. Let us know if you need further assistance.