Navigation is controlled by an XML file that can be accessed by going to Settings | User interface | Navigation menus:

There can only be one active navigation file per app, and it is always called default.

After clicking on the name, we see the XML provided by the barebones template:

<nav search_view="search" color="#65A637">
 <view name="search" default='true' />
 <view name="data_models" />
 <view name="reports" />
 <view name="alerts" />
 <view name="dashboards" />
</nav>

Note that if you check the navigation for another app (search) you will notice the same XML.

The structure of the XML is essentially the following:

nav
view
saved
collection
view
a href
saved
divider
collection

The logic of navigation is probably best absorbed by simply editing it and seeing what happens. You should keep a backup, as this XML is somewhat fragile, and Splunk does not provide any kind of version control. Here are some general details about nav: