How to open a circuit breaker

Once inside your box you should turn off the breaker being changed,There will be a black wire coming off the breaker - you need to loosen the screw and remove this wire. The wire goes to the circuit branch in your house that is controlled by the breaker. Set this wire end aside - it is not live but don't let it contact anything less you accidentally make it live!

The breaker is plugged in to  a rail in the bottom of the breaker box, the rail is live. You have to release breaker which has a detent that locks it to the rail by tilting and removing the breaker.

You then need to put the replacement breaker - must be the exact same rating or you risk burning down your house! - Make sure its off, then tilt it in and snap-lock it in place. Now replace the black wire to the circuit branch and turn the breaker on.

If you want to be extra safe, turn off the master breaker at the top before you change anything, then turn it back on after you've changed the breaker.

remember there will be some exposed electrical contacts when you do this. You'd better be able to recognize and avoid them.