Please review the operators manual for instructions on how to release the transmission so that you can roll the unit by hand.
If this is a tractor style with a hydrostatic transmission, there is a single lever that allows you to activate the dump valve, thereby allowing you to push the tractor.
If this is a "Z" style mower with hydrostatic drive system there will be two releases that will need to be activated before you will be able to roll the unit.
NEVER attempt to TOW the unit, major transmission damage will occur.