If you do not have an owners manual, go to TORO's web page and download one . They are free.
Older ZTR's had a bypass valve with a lever or rod on them.
Newer models just have a nut, one on each motor or pump so there will be 2 on your mower.
Usually it has a funny looking head so when you look down there will be dozens of plain hex headed bolts then one with slots or dimples, this is the bypass valve.