It`s very easy, just unscrew the four screws that hold the PSU to the case,disconnect all PSU leads from motherboard and hardware like HD etc,then remove the PSU,refit new one,connect leads etc all done.If you`ve a floppy drive make a note which way round the power lead from PSU to floppy drive goes on,since fitting this wrong will result in dead floppy drive(happened to me once 😉).