I had the same issue and it was driving me crazy… After searching online and in here, I was able to resolve it. To edit config.txt, I shut down my pi and pulled the SD card and opened it up on my PC. From there I was able to open config.txt and at the bottom of the file I simply added
## temp0 is lower speed fan, temp1 is high fan speed.
## Current setting is fan turns on at 70C, turns off when it comes back down to 65C.
## If the temperature reaches 75C it will go full blast until it drops back down to 70C.
The fan hasn’t kicked on since I powered the unit back up about an hour ago… but I’ll probably play around with these settings slightly after monitoring the temp for a while.