Connect projects to Persons and ACF

I’m trying to make this work with Advanced Custom Fields but I got stuck. I need to change the person related to a project from the front-end. I have a custom field that is getting the list of persons but can’t figure what to do next.

It’s the last field on this page (Op.): https://goo.gl/0AIyNa

https://www.ads-software.com/plugins/orbis/