The Replace Existing Device option can be located by choosing a device and right clicking on it and picking the "Replace Existing Device With This" menu option. This item will bring up a list of your devices and let you pick one; the device you choose in this list will be replaced by the one you right clicked on to bring up the menu.
This can be useful if you've lost a device and need to replace it. By doing so, you'll be able to keep the settings, name, scene placements, triggers, etc. that were part of the original device.