How to get an asset URL into a NAV link

This is probably the most basic of questions, but I am new! I have uploaded a PDF into my assets that I would like to replace an existing PDF with. There is an old Navlink URL that would like to replace with a new URL from my new asset. I have selected my new asset, and copied asset link to clipboard. Now how do I get that new URL over to my Navlink?

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