The config.txt is located in the Boot folder, you can edit this via ftp or by putting the SD card into your PC. I had an issue with locking up as well when trying to overclock my rpi3b with the recommended overclock settings I've seen on several sites. I ended up just reverting to default. it's always a good idea to make a backup of your config.txt before editing just in case something like this happens. hope this helps.