You can find the navigation settings on the left side on the powerbar, by clicking on the gear icon > “navigation” and the navigation settings will open. Learn more about it here

If the option “Navigation” does not appear, please make sure that you are in the guideline and not on the cover page. The navigation settings are only visible from the guideline.

Here you can see an example when you open the settings on the cover page:

Click on another item in the top menu which is not the cover page and the option should appear now:

If the powerbar on the left doesn’t appear, it probably means you only have “Viewer” rights and therefore can’t open the settings and edit them.