The height of a CO detector isn't very important because CO is a diffuse gas and therefore spreads evenly in a room. CO is about as heavy as air.
This means that you can put the CO detector at 'sleeping height' in the bedroom and at 'sitting height' in the living room. In any case, choose somewhere clean, away from children and pets.
Always read the manual for the specific installation requirements. Some CO detectors also have a stand so you can easily place it on a cabinet and do not need to mount it to a wall or ceiling.