With my single button Meizu M2 Note (Android 5.1) and no 'go back' button, it's very difficult to exit some of the sub-menus.
E.g. 'Settings' from Main Menu and 'Download Maps' from 'Map Manager' from Main Menu
The only way I've found so far is to swipe-down from top to get task bar, and then select 'MapFactor Navigator - touch to return'
Other screens get around this in various ways e.g. graphical map display has triple-bar option button bottom left, 'Tools' from Main Menu has (touch outside sub-form), 'Speed warning threshold' has 'Cancel/OK'