Well obviously you can drain the fluid & refill because the manufacturer had to fill it in the first place.
There is no drain plug on bottom end trannies
Most tranny makers have manuals & parts break downs on their web site However if the motor or pump is badly scored then changing the oil will be of little use.
Mower companies like to fit "sealed for life" transmissions because if some thing goes wrong with them , it is not their fault & the oner has to take their problems to the tranny makers.
Also they don't need to keep service parts for them .
Most will have a vent some where on the top & on the cheapies this unscrews and will drain the oil out when the unit is inverted , although it might take all day and refilling can take longer .