I think i had the same about not getting that i had to click on the necromancer, at the start… fair.
To make a level 3 unit, you simply drag and drop a level 2 unit on another level 2 unit of the same type, just like you would do with level 1 units. The only thing you have to mind is that they must not be carrying different pieces of equipment – a unit that carries nothing can be combined with one that carries a shield, for instance, but if one has a shield and the other a funny bone, you cannot merge them.
Also, I am not sure about that but i think if you want to keep the equipment, you have to drag and drop the unit that carries nothing onto the one that carries the equipment, otherwise it seems the equipment is lost. Same if you have a unit that was buffed with milk or a building: if you want to keep the buff, drag and drop an unbuffed unit onto the buffed one, not the other way around.
You cannot simply add a lvl. 1 unit on a lvl. 2 one, they must be of the same level to merge. So in the end, you'll need 4 identical lvl. 1 units to make a single lvl.3 one
