Settings>Services>Web Interface>click on reset to default then reboot. You can go back with your mouse by right clicking on a blank/inactive part of the screen. There is also a “Home” button at the bottom right corner of the screen (using Confluence skin, not sure about the OSMC skin) you may need to enable mouse/touchscreen support in Settings>System>Input Devices to make the button show.
Just to clarify choosing reset in the web interface screen will only reset that page of settings to default, NOT the complete system.