Bosch is a manufacturer of many corded and cordless tools. They make several different sizes of hand drills that all have one thing in common. They all use a Jacob's style drill chuck. A Jacob's drill chuck uses three teeth to grip a drill bit. If one of those teeth break, the drill no longer has the capability to grip the drill. If this happens, you must remove the old chuck and replace it with a new one.
Place the drill into reverse with the button located by the trigger. Hold the chuck still while you pull the trigger. The chuck will open up. Do this until the chuck opens all the way.
Unplug the drill chuck from its power source or remove the battery. This will prevent the chuck from accidentally turning on while you work on it.
Locate the Allen screw located in the inner rear of the chuck. Remove the Allen screw with an Allen wrench. You may have to hold the chuck still with one hand while you remove the Allen screw with the other.
Plug the drill back into its power source and then pull the trigger while you hold the chuck. This will unthread the chuck form the driveshaft.
Disconnect the drill from its power source again.
Thread the new chuck onto the shaft with your fingers. Do not power on the drill to do this. It could cause you to cross thread the chuck onto the driveshaft.
Secure the chuck to the driveshaft by inserting the Allen screw and tightening it.