Hi, I think you need to edit config.txt located at /boot/config.txt

Have a look at the options you can configure here: https://github.com/Evilpaul/RPi-config/blob/master/config.txt

Specifically sdtv_mode, sdtv_aspect, hdmi_group and hdmi_mode. HDMI will override composite if the Pi detects a HDMI output, but when only composite is connected the Pi should read your specified output settings from config.txt