How do I destroy my old level 1 level 2 soldiers?

Unfortunately the game no longer allows you to remove healthy troops. The only way to remove them is when they are wounded. Although some battle will also kill them permanently.

Assign your unwanted troops to the front line of an APC and make attacks against any target to let them die or be wounded. If they are wounded and end up in the hospital you can dismiss them from here which removes them permanently.

In the hospital interface you need to go to the Heal option but instead of selecting them to heal, tap on the troop icon to the left of each troop type and you will get an option to dismiss the troops.