Very easy, firstly i presume it has a pump in the back end, if so take the drawbar of or lower the pick up hitch.
Then you will see a round plate underneath at the back of the tractor, in the plate you will see a large plug, that is a drain plug for loosing the oil out, maybe a 27mm if my memory is correct.
Once the oil is drained undo all those bolts and remove them all except 1
then get as far a way as possible and just slacken the last one a few turns and let the remainder of the oil drain out.
When that has happened take that last bolt out and put that round plate on the work bench, there is your hyd pump sat in the middle of it, it should be plain sailing from there on, just remember to renew the o-rings on the stack tube.
if it is a front mounted pump (mounted at the front of the engine) and you need advice on how to change it, then you are not safe to be holding a spanner
good luck
O