I don't think it is actually possible to change troops. What you can do is to click battle and click on the campaign adjust drop your army if you have already completed your campaign. If you haven't completed the campaign then you might just want to use those troops to attack and beat the campaign. If you are worried about losing the food it took to train your army don't have enough to make a new army. Don't worry too much the animals can be hunted and you can get your food back quick. Also if you have farms you can get enough food to train your army quick. If you have more then enough food then don't worry about it and just attack the campaign.
Just my 2 cents
