1. Shut off the engine and let the mower cool down. DO NOT attempt to start it again.
2. Find a safe place to park the mower, away from any sources of ignition. The fumes from diesel fuel are flammable, so it's important to do this to avoid any accidents.
3. Call a qualified mechanic to come and take a look at your lawn mower. They can safely remove the diesel fuel from your mower and repair any damage that it may have caused.
4. Prevent this from happening again in the future. Be sure to clearly mark your gas and diesel fuel containers so that you don't accidentally mix them up. You can also install a fuel shut-off valve on your lawn mower so that you can turn the fuel off when you're not using it.