Is there any way to order my troops to retreat when I'm already in the battle? Or is the only way to forfeit the whole battle?
If you use the "withdraw" option for your units and can get them all off the field (They will head in a specific direction, usually the side you arrived at) then you will have retreated, this is slightly more preferable to "forfeit" as it will ensure that you don't immediately concede and have casualties calculated.
However you can only do this in certain circumstances. When defending a city you can't and if you have been attacked and don't have enough movement to retreat (ie you were given the option when the battle started) you can't retreat either.
If you are an attacker you can always retreat.