Undo all the bolts that join the two units together, take off the clutch flywheel etc etc etc and LIFT using the crane or whatever you used to get it out of your car. The engine should go up and the gearbox should stay on the ground but do it slowly in case there's something that you forget.
When it comes time to put it all back together, don't forget the all important rubber O ring on the face of the gearbox where it seals with the underneath of the engine, or you will have a very unpleasant surprise when you start your car
Have fun and btw, how did you source another gearbox???
Cheers