I want to change out the hydraulic oil in my system. Not sure where the drain is, I don't have the service manual.

You really need to get access to the ACMOC library or buy the OMI for the NO. 46 hydraulic control, but here you go.
Craig
 
 

I found a detailed procedure in the Service Manual.
Drive ttractor so front idler is resting on a 10 in timber.
Black the back so the tractor can't roll back.
Raise and block the blade to it's highest postion.
Remove the two drain plates on the bottom of the tank and drain the oil
Change the filter
Once drained lower the blade by removing the blocking ( I would assume some type of lifting equipment is called for or the blade would just drop uncontrolled)
Replace the drain covers and fill.