How can the previously created form be edited?

How can I submit a form that I previously filled out but now I want them to acknowledge and sign

As far as I understand your question, I can suggest that you want to edit your previously created form.

To make changes to your previously created form, you need to find it at your "My forms" page and click Edit.

As for the other part of your question 

I want them to acknowledge and sign

I can suggest you to look through JotForm's signature widgets to choose the one more suitable for your purposes:

You might also consider adding Terms and Conditions widget:

Here is a useful link about adding widgets to the form:

How-to-Add-a-Widget-to-Your-Form

How-to-Add-an-E-Signature-to-Your-Form

Hope this helps!