Where is the bolt to change the oil found?

Need more details

--- oil plug on push mower under the mower,
Bottom of engine

--- riding mower probably has plug in side of motor,
Near the frame, one side or other

If it is a rider, look on the left hand side of the mower for a pipe coming out horizontally, there will be a cap on the end. Or if it is new enough , there will be a yellow cover that you remove, then push in and twist the cam lock to be able to pull outwards and drain the oil.