How do I edit/modify a form that was originally inserted into a web page?

From your request, I think you want to know the form that was embedded in your website. I presume that you do not know the form URL/ID and that is the reason for this query, right?

The link that you have shared is not accessible. Please share the correct web URL from where you want to know the details of the form.

 The link to the form is https://swsc.co.uk/classic%20results%20form.htm

I am using Xara web design premium Ver 15 - although this is not supported by Magix.  I have paid for and tried to update my version to Ver16 (which I have done) but I cannot acess my previous web site file - error messages come up.  Magix are looking into this for me but at this present time I have to use Ver 15.  I did not originally write this web site and have taken it over recently - hence I'm still trying to find my way around it.  I composed the original web site using Serif software so this is quite a change!

The form that you have embedded into the web page URL that you have provided is: 

https://form.jotformeu.com/80962434084358

If you do not own this form, you can clone it and make changes to this form and re-embed on your website using the iFrame code - Getting-the-Form-iFrame-Code