How can I customize the top nav of model driven app.
I need to customize the top nav and add a link or button.
That's the command bar (previously known as the ribbon bar).
There is a decent bit of documentation at Customize commands and the ribbon.
You effectively need to download the Dynamics solution and edit the XML of the solution.
Both the command bar and the ribbon use the same underlying XML data to define what commands to display, when the commands are enabled, and what the commands do.
However, there are community tools that make this process significantly easier - Ribbon Workbench.