How can I change the chuck on my Ryobi 430W drill with cord?

I'm having trouble removing the chuck from my Ryobi drill (about 10 years old, 430W, model details and manual lost, 3/8 inch chuck key, chuck made in Germany, maximum bit capacity 10 mm, with electric cord). In the centre of the chuck (when it is wide open) there is what looks like a screw head with a large round dip in the middle. My screwdrivers are useless on it and I suspect I'm doing something wrong. Can you please help?