If you mean recall from another city (that you've supported) you click on the agora in your city, then click on the 'Outside' tab. From there you can recall them.
If you mean recall from an attack, then you must scroll over the little sword emblem on the top left of your screen, beside the little map in the corner. From there (within the first 10 minutes of sending the troops) you can recall them from the attack. After 10 minutes have passed after sending troops, you CANNOT recall them, so bear that in mind.
