Hi, i have an Arduino UNO Rev 3, accidentally a step-up and regulator board, has fallen over my arduino board while running, and it made electric contact with one of the ICSP headers, then i pulled it away as fast as i could but the mega 16u2 was already hot and smoking 
Now i cant upload new sketches, RX and TX leds are always ON, i dont know if the ATmega328 is ok but i got a few of them so thats not a problem.
the step up / regulator has a max. voltage of 12V in its circuit.
https://scontent-mia1-1.xx.fbcdn.net/hphotos-xap1/v/t1.0-9/11866225_10204568548589872_7700851481884503156_n.jpg?oh=1e276775ef246a0afd7119d5a0447aa0&oe=563BA6E5
How can i know how damaged is it?
Can i replace the mega 16u2 if it is destroyed or should i buy a new Arduino?
Thank you! 
Can i replace the mega 16u2 if it is destroyed
Only if you have surface mount rework equipment, like a hot air rework station.
Phisically replace it is not the problem, i can get my hands on SMD equipment!
The real deal is: Where can i get another 16u2 to replace it?? (same program and function)
How can i know if the only damaged component is this microcontroller?
The LM358 comparator may be fried also.
Not much else runs from 5V (as there isn't much else). Maybe an LED got zapped, or the p-fet that supports the autovoltage selection.
Replacement parts from digikey.com, mouser.com.
Code is available from Arduino website, or github, might even be part of the Arduino s/w distribution.