Hello,
I need your help!
Accidentally I have applied to my arduino Mega 12 volt voltage and now I can not upload more programs.
What happened there, I think it is a small chip broken, :o
and who can I replace him? 
Where did you apply 12 volt?
At "Vin" or "power supply plug" 12 volt should not be a problem.
If you have applied 12 volt to pins "5V" or "3.3V" you may have killed the mega.
12 volt at Input/Output pins may also have killed it.
It may also depend how long 12 volt were applied. ATmegas are pretty robust and have survived when I was abusing them pretty often, if the "abuse" only was "short enough" (a few seconds) 
I recently applied 12V to the 5V of a mega because of an accursed non-standard adapter that produces 12V on a USB connector. I pulled the power when things didn't work but it was on for a few seconds. But it still works now, at least for the time being...
Basti5559:
can I replace him?
The ATMega2560 is 100 pins SMD, if you can replace it yourself.
Replacement mega clones from eBay starts at about $8 if you don't mind long wait during shipping. If you need to repair the board, you need someone that has proper SMD work station and has experience with them.
I checked Digikey and Mouser, the chip alone is $10 to $12 in single quantity. IMO I wouldn't bother to fix the board unless it's official Arduino brand and not clone.
aarg:
I recently applied 12V to the 5V of a mega because of an accursed non-standard adapter that produces 12V on a USB connector. I pulled the power when things didn't work but it was on for a few seconds. But it still works now, at least for the time being...
Whatever puts out 12V on a USB connector should be crushed now! Few seconds doesn't matter,
over-voltage damage to CMOS happens in a fraction of a microsecond.