Click on the heroes icon at the bottom of your screen. The button to access the heroes in is located on the top right above your helmet.

You can only have 12 heroes at a time in your lower inventory so if there is not room to drop a hero down you will first have to click on one of the heroes your not using and make them rest in the heroes inn. The rest button is in the same place depending on where you are (just above helmet).

If you have room to drop a hero out of the heroes in just click on the hero inn button mentioned above, scroll along to find the hero you want and hit the deploy button located on that particular hero.