Updating an entity using posted form fields

I have a web content form set up we want users to fill out, with the end goal of updating a separate entity with the information. The entity is not related to a contact or lead. When the form is submitted it will contain a hidden field with an ID that I want to use to lookup a specific entity, then I want to update fields in that entity using the form fields. 

I am trying to set up a workflow for this, but can't see how to get from the hidden field ID to the entity I want. I only see how to create a new entity.

Can someone help me figure out how to do this?

Thank you!