You might not be able to due to being a sealed unit.
Some have a fill plug but no drain plug so you would need to remove the transmission and turn upside down but not gauranteed to get all the fluid.
What transmission is it?
Even in a sealed unit changing the hydro oil shouldn't be too difficult, just maybe a little time consuming as it's best to remove the transmission & invert to drain- no matter what brand the transmission is, there'll be a filler threaded "plug" or push "cap" that was used at the factory to initially fill the unit (& can also be used to drain the unit as a invariably a sealed unit doesn't unfortuntaely by design have a drain bolt/plug ) - as alternative to removing/inverting the unit you can use a hand powered oil suction gun/pump (usually found in auto or marine shops) to remove the oil via from the filler plug/cap however this method is as effective as it doesn't always remove all oil/gunge.
It's good maintenance practice to change hydro oil & in my JD LT155 which has now done close on 1100hrs ( the tranny performs as new) every 100hrs I replace the oil with premium synthetic product (the TuffTorque tranny supposedly is sealed for life too but the way I see it : oil breaks down, & all hydrostatics hate heat & degraded/dirty oil so it's cheap insurance & relatively easy to do on this so called sealed unit) & ditto for a sealed Peerless tranny I have in a homebuilt GT - the trouble is by the time you notice diminished performance in a hydro due to oil breakdown it's usually too late to recover the unit by an oil change.
When changing Hydro oil:-
- clean the unit thoroughly with compressed air or wash in water/dry prior to removing any filler plugs/caps, with hydro's any dirt ingress can quickly kill the unit
- the oil level is critical, so measure/mark & replace exactly how much you remove (there may be a fill level marked just inside the filler housing but it's not always there)
- there may be a small filter mesh screen & possbly a small spring just inside the filler housing which can be cleaned too is it drop out.
- take time to let the oil drain slowly to empty as much as possible
- also take time to fill with oil to allow the oil to distribute evenly in transmision & fill slowly to minimise any air bubbles/air locks forming in the oil, & also occassionally whilst filling rotate the axles slowly by hand to distribute the oil evenly in the hydro unit
- depending on the filler plug/cap fitment, once you replace firmly it may be prudent to use a small bead of silcon externally around plug/cap to exluded all possibility of dust ingress.
