Drop your drawers, hold your nose bend over and blow hard.
The above of course assumes you sat on it & it is stuck up some where that the sun never sees.
As for replacing one on a mower, it would help if you told us the make & model of both the mower & the engine.
Also weather it has a remote solenoid or has one on the side of the starter.
1. Disconnect positive battery cable from the battery.
2. Mark wires so you will know where they go when you put them back on.
3. Take pictures. So you will really know where they go.
4. Remove nuts holding large wires. Remove wires.
5. Unplug two small wires.
6. Remove two bolts or nuts holding solenoid on.
7. Remove solenoid.