To edit existing waypoint either tap on it on a map and pick "Open" or use MENU > WAYPOINTS to find the waypoint and tap on its row to open waypoint editor.
Following waypoint properties affect the alerting:
Use the map view (tap on a blue MAP row header) to have visual indication of the approach angle and setting it:
When in the map view, tap on a left-center button to enable the angle editing circle (gray one). Once you see this gray circle around waypoint, tap on it to adjust the angle.
This way you can edit waypoint/camera alert properties.
Related information: