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Edit a collection items from a webpage

I have a site with a collection of contacts. I would like to add the option for my visitors to be able to add new contact and edit existing contacts.

So far I have managed to do the first part by creating a form and link the form with Zapier to the collection so when a new form is submitted it add a new collection item. But I cant figure it out how to update an existing collection item. I see that in Zapier there is an option to update collection item but I figure out how to get it work. If I use a form again - I would need the form to be prepopulated with the information of the collection item, then to make the changes and make the zap, but I cant see how I could do that?

Any ideas how I could do that? I am open to any kind of solution, it doesnt have to be related to Zapier or forms just couldnt figure out another way.

Here is my site Read-Only: LINK
(how to share your site Read-Only link)