Not completely sure of your deck... but they are all pretty similar. A picture would help.
But basically take the blades off.
Remove all the bolts attaching to the gearbox, few on top, there might be a few underneath the deck as well going up
Then underneath I think the 'cap' will pull off and there will be then be a big circlip(s) that you need to remove from the bottom.
Then tap the big shaft through from the bottom which will also push the gearbox up.
There will be lots of grease when you take the shaft out.
Make sure you put it back together the way it came out!
Good luck, its really not that bad.