It's much easier and quicker than you think.
I have the L3940. The others are right that there is no need to change fluid until 300 hours.
Here's what I did and only lost about a quart or less of fluid. And I put new fluid in to replace the lost. I don't like reusing fluid due to possible contamination (just me).
There should be one filter on each side of the tractor just under the foot rest area. One is for the HST and the other is for hydraulics. If you have a gear tractor there may only be one filter. Anyway, put a CMU block (cinder block) in front of the front tire and in front of the rear tire on the side that has the filter you want to change. Drive the tractor up onto the blocks to raise one side. Remove that filter but have the new one ready to go on. You would be amazed at how little fluid comes out with the tractor lifted only 8" high on that side. Put new filter on, start tractor and drive off, move blocks to the other side...."rinse and repeat"