To train troops, tap on a landing craft, then select the add troops button and then choose "Reinforce". Choose the type of troop you wish to train, or reinforce your depleted squad. Training time depends on the troop (eg: Heavies take longer than Riflemen).
You can also reinforce all your landing crafts at once by tapping on the grey square troop icon on the right side of the screen.
If you need to change your troops, tap on a landing craft, then select the add troops button and then choose "Change troops". Your current troops' cost will be refunded.
In order to make your troops stronger, you'll need to visit the Armory to upgrade their equipment.