What should I do after topping up the oil in a lawn mower with petrol by mistake
Drain the oil. 
Refill with new clean oil.  


Check the manufacturer's documentation for oil change procedure, and type of oil to add. 



Looks like the drain plug on your mower is just below the oil filler tube.


  


To change the oil:


Remove the spark plug cap.
Tip the mower so that the oil can drain without flowing across the deck (Make sure the mower is secure before proceeding).
Put an oil pan under the drain (use a funnel if necessary).
Remove the drain plug (be careful not to drop it in the oil pan).
Allow oil to drain.
Replace drain plug.
Set mower flat.
Fill with recommended oil to fill line indicated on dip stick.
Wipe up any spilled oil.
Replace spark plug cap.


TIPS: 


To prevent accidental starts, always remove the spark plug cap before working on a lawn mower.
If the drain plug has a gasket washer on it, moisten the gasket with a bit of new motor oil before installing the plug.