i have a roof tile that as broken?

If it's a plain clay 10.5ich x 6.5inch easy they have 'Nibs' on the back that hook over the tile battens [in fact all clay or concrete tiles have 'Nibs' the only ones that are nailed on every row are slate roofs, and very low pitched interlocking tiled roofs. to hold them in place. Every 5th row will or should be nailed as with the top and eave tiles.
Lift up the two above with a pointing trowel and turn them sideways as far as you can to clear the broken tile.
It may be nailed so pull it up off the nails and remove those nails before putting the new tile back better you don't nail it you may break it.

Worth remembering metric tiles a slightly small than old imperials so if you replace many don't do more than six together mix them up with old and new to break up the very large gap you will end up with if you bunch say 50 in one area.
As a former jobbing builder who has repaired many roofs you can if its not felted or a boarded roof repair some not all from inside the loft. Any broken over the rafter remove those between and slide out the broken bit replace that with an old one you took out followed by the rest leaving the centre between rafters last to be replaced.

rob
April 2007