JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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I'm not sure if I understand your query correctly. Do you want to update a form which is already published on your web page? If so, you may update the form from the form builder even if the form is published on your web page. The changes made to your form will be automatically reflected on the published form. However, if you have embedded the form using the Source code, you'll need to re-embed the form on your web page. Otherwise, it should be updated automatically.
Hope this information helps! If you are referring to something different or need any further assistance, please let us know. We will be happy to assist.