upload sketches to mega using uno

Hi any help would be greatly appreciated. its been two days and i still cant get it to work

I have an arduino mega that none of my computers detect no matter what i do. ive installed drivers (clone drivers, ftdi drivers etc.) , tried other operating systems (win 10 and 7) and nothing.

I have an uno that connects just fine and would like to use it to upload sketches to this mega, but cant get it to work. it times out all the time. i uploaded nick gammons sketch of arduino connected it via the pics and uploaded via programmer selecting arduino as isp. still it times out. i know it works because it runs the previous sketch (marlin). maybe im doing something wrong or missing something, cant think clearly so if anyone knows how to and can help please do so!! Thanks in advance