how do I edit an existing form

Log in into your account, click on MY FORMS, select the form, and click on Edit Form. 

If you want to use and edit a form from the templates gallery see this guide "How to Use the Form Templates", and if you need to clone an existing form see this guide "How to Clone an Existing Form from a URL". Hope this helps you, let us know if you have more questions, we will be glad to assist you.