The option to transfer is listed on the far right side of the army sponsorship page. (The same screen where you have links to change marshal, add funds, etc.) If you are the sponsor of an army, and you are NOT the lord of the army's home region, there should be a Transfer link. The option only appears if you are not the lord of the home region, and it will only transfer to the lord of that region. It is not a general "transfer to someone else" option.