Some TVs do not have the back button correctly mapped by default in Kodi (via CEC over HDMI), this makes navigation of the menu structure very difficult. However, a fall back can be created by mapping 'Back' within Kodi to another button that is not used as often on the TV remote. I selected the Red button for this task.
To do this, SSH into Kodi using the user root (default password 'openelec') and create a file named '~/.kodi/userdata/keymaps/samsung.xml' with the content
<?xml version="1.0" encoding="UTF-8"?>
<keymap>
<global>
<remote>
<red>back</red>
</remote>
</global>
</keymap>
If you wish to correctly map the back button for your TV, you will need to first discover the correct code that is assigned for your TV. This code can be found by enabling the debug mode with the settings menu of Kodi, and then monitoring Kodi's log file.
tail -f ~/.kodi/temp/kodi.log
For each button that is pressed on the remote, a log entry will be outputted with the matching key code.
18:40:18 T:1868559424 DEBUG: CecLogMessage - >> 01:8a:91
18:40:18 T:1868559424 DEBUG: CecLogMessage - >> TV (0) -> Recorder 1 (1): vendor remote button down (8A)
18:40:18 T:1868559424 DEBUG: CecLogMessage - key pressed: return (Samsung) (91)
18:40:18 T:1868559424 DEBUG: PushCecKeypress - received key e5 duration 500
18:40:18 T:1969861152 DEBUG: OnKey: 229 (0xe5) pressed, action is Title
18:40:18 T:1969861152 DEBUG: CGUIWindowManager::PreviousWindow: Deactivate
From the above you can see that key code 229 was pressed, which is currently mapped to Title. However I wish to remap this key to Back using the following keymap XML
<?xml version="1.0" encoding="UTF-8"?>
<keymap>
<global>
<remote>
<red>back</red>
</remote>
<keyboard>
<key id="229">back</key>
</keyboard>
</global>
</keymap>
You can find more information about keymapping on kodi.wiki