How Do I Change The Drill Bit On My Bosch Psr 1200 Drill?

  I have a Bosch PSR 1200 drill. I was given this and it has no instructions.
  Please can someone tell me how to expand the place where I insert the drill bit (it has 3 speed in it, I think it may be called "Chuck"?) Then again tighten around the fuse please?
  Thank you in advance