If you look at the two horizontal levers on top you will notice that there is a pin which prevents these levers from being squeezed together. While the pin is in place, there is little risk that the extinguisher will discharge accidentally. The lower lever is intended as the carrying handle.
To discharge, first break the tamper seal which holds the pin in place, then remove the pin. When you squeeze the two levers together, the extinguisher will discharge if it is fully pressurized.