It's not too hard, as long as you have done rework before.

The easiest way to get rid of the old display is carefully cut all the pins off, well away from the PCB. Remove the glass. Next, pin-by-pin, remove the dead legs from the PCB. Finally, use some solder braid to open all the holes again before dropping in the new display.

Trying to remove the display "intact" is a lot harder and makes it easier to damage the board.

It's useful to solder just one pin at each end first to get the alignment right (and ensure that both ends are mounted with the same amount of pin going through the hole!). If you have a mk2 (as opposed to mk2a) one pin needs bending back and soldering to a patch wire that you should have noticed when removing the old display.

Hugo