How do I change the screw on my Spear & Jackson electric screwdriver?

I've never seen that make, but I believe the shank is magnetised, so the bit should just pull out.
If a screw has been driven home tightly the bit can sometimes get caught in the shank. Put the lock on, grab hold of the body of the bit with a pair of pliers and give it a very slight turn anti-clockwise.

Big Softy
April 2007