To level your troops, you need to go to barracks, that are unlocked after reaching Castle level 8. There, you will find your troops list. Select the one you want to upgrade, and then select the troops you want to feed. It will require a lot of FOOD, be aware and try to level it wisely. Start with low level star troops for leveling and save the high star (3*/4*) to level when your troops reach level 20+