Hello, I bought an Arduino Mega (2560) and, when I connect it to my computer, as an Arduino Uno 
The Arduino IDE version is 1.6.12.
Is there a way to solve this issue?
What exactly do you meant by "recognized as Uno"? Are you just talking about the label in the Tools > Port menu?
Yes, in the submenu port on Arduino IDE.
The problem is they set the wrong PID in the firmware of the ATmega16U2 chip on the Mega. I don't believe this will cause any problems for using the board. I would just ignore it. If you do want to fix it you would need to flash the ATmega16U2 with the Mega firmware. I know there are two different methods for this, DFU through the USB cable or by connecting an ISP programmer to the ICSP header next to the ATmega16U2 (not the one next to the ATmega2560). I have never attempted either of those things but there are some tutorials available. the firmware can be found included with the Arduino IDE in the hardware/arduino/avr/firmwares/atmegaxxu2 folder.