I recently bought this router from my ISP and surprise it has their logo and limited menu and what is worst in system  has no firmware update button, is completly removed.

Version 1.2 firmware available and i canot flash it because idk how since is not in the menu. I do not care about warantly since i allready paid for it, i just want it updated cus new firmware fixed a bug with 2,4 ghz band. 

How can i flash it? Is there a flash tool to bypass that stupid custom firmware?