The height of the placement of a CO detector is not of great importance because CO is a diffuse gas and therefore spreads evenly in a room. CO is about as heavy as air. So you can easily place the CO detector at "bedroom height" in the bedroom and at "sitting height" in the living room.
In any case, choose a place where the CO detector stays clean, away from children and pets. Always read the manual for specific installation requirements. Some CO detectors also have a stand, so you can easily place them on a cabinet and don't have to hang them from the wall or ceiling.