Hi, looking at the controls you have the tuning knob on the far right hand side and to the left of that is the function switch with 3 options 'CD, OFF, RADIO' this is the on/off switch.
Most of these cartridges either clip in or are held by a small screw somewhere. You will need to remove the connection wires first. Make a note of which coloured wire goes to which pin. Some pull off with the fingers. Others need small pliers to remove them. Use only gentle pressure.