Cannot unrestrict pdf form I created (no password) therefore I cannot edit - please help!

Hi.  I have created a pdf form and saved as restricted (with no password), to enable it to be sent out and filled in.   I need to make changes but cannot find a way of unrestricting the form to enable editing.  When I open the document it gives the message "this document restricts some Adobe Acrobat DC features to allow for extended features in Adobe Acrobat Reader DC. To create a copy of the document that is not restricted (and has no extended features in Adobe Acrobat Reader DC), click Save a Copy."

When I try to save a copy it tells me that I cannot save a copy that is not restricted.

I have previously edited this form by simply switching off the restricted settings but cannot find a way of doing this now.

If I go to the security settings it tells me I cannot change them because the document is restricted.

Please can anyone help me understand how to unlock this form and save me having to do the whole (very time consuming) thing again?  It seemed so easy at first!

I am using Acrobat Standard DC 2015.

Thank you.