How do I create my own tabs, or edit the existing tabs heading or image?

At the bottom of the screen area that shows the thumbnails of the page are three little icons.
The first shows you the opening title page.
The second the slide layout view. You can edit the basic slide template here and add or delete elements to make custom layouts.
The third is the default slide layout view.

on the navigation objects, there are only default tabs e.g. contents, settings, how do I create my own tab or edit e.g the settings tab to read e.g. objectives tab?

All of these are kind of special objects often quite composite in their construction.
You can replicate some of the more obvious like the fly in tabs, some have custom code associated with them and are more difficult to change.
You can change the appearances by finding and changing the base images.
So if you wanted to make a tab type button either modify whats there or make the tab graphic and apply relevant actions to get the effects you need.