Two options -
1) Go to call of duty ghosts app, select my clan, there should be two helmets in the top right corner, click on the one with arrow, it will ask if you want to leave clan, select yes
or
2) Send and application to the new clan you want to join and get them to accept, it should automatically make you leave your old clan
If you don't see the two helmets on the clan page you may need to clear data and cache for the app, then re-sign in, failing that unistall the app then re-install, hope this helps.