How do I edit Navboxes?
Find this option via: Menu > Settings > Navboxes.
From this dialog displayed below you can select which of the available Navboxes you wish to display on the active map via the tick box on the left of the Navbox name.

Highlighting the name of the Navbox in blue allows you to Edit the way the box is displayed via the Edit button.
Navboxes are configured separately for Map 1 and Map 2 views. This is because you will normally want to use the two map views for two different reasons. For example, one view could be used for general navigation while the other one for approaching a waypoint where specific details are more important.
Note: if shutting down with the power button you must save settings for your changes to remain in effect for the next time you start the device. Find this option via: Menu > Save Settings.