If you're using the box image from an SD card the boot params are in uEnv.ini. If you installed LE to the eMMC storage then boot files are a bit different and there's an extlinux.conf file. However, configuring (forcing) an HDMI mode will do nothing when you're connected to the TV with a composite cable. You can try the following but I doubt it will work: video=Composite-1:1920x1080M@60
Use an HDMI cable and you'll get proper modes.