How to clean a burnt wall socket?
Do not try to clean a scorched receptacle even with the power off; the heat created takes the temper out of the metal and the receptacle would not pass a withdrawal force test, this is important because it measures the contact strength and overheated receptacles need to be replaced not cleaned.

A cheap replacement may be purchased for around 1 dollar, but a quality spec grade receptacle around 4 dollars.

I install hundreds of receptacles and I always purchase back &amp; side spec grade (and even heavy duty industrial for up to 7-8 dollars each).

Why would an electrician spend so much more on parts? They are safer and last longer, yes contractors going in and finish wiring tract houses use the cheap ones in most cases but not all.

Changing a receptacle only takes a few minutes and is a simple skill all homeowners should be able to do:

Take a photo before starting, ask any questions before starting.

Turn the power off at the circuit breaker, 
1 or 2 screws removes the cover plate.

2 more screws releases the receptacle, worst case now there are 5 more screws

2 silver screws with white wires, 2 brass screws with black wires and 1 green screw with a bare copper wire.

Swap the receptacle and then put it back in and cover and you are done, turn the breaker on and you have it finished.

Please do it right or get help, it is really not that hard.