Skinning is the simple answer, but if you're just looking for some basic changes, then the Art - Tinting and Art - Custom Team Colour fields in the object editor might save you hours of trouble. They let you change the overall colour of a unit, tinting it, say, red, or make the unit appear permanently on a certain team (team armbands or whatever). If you've ever played DotA, then I can safely tell you that no unit in DotA required skinning, but has simply been tinted. If you actually want to change parts of the units, then you're stuck with skinning, but hopefully you won't be getting into modeling. Skinning is changing the unit's 'skin', the outside. It's the armor and face of the Footman. Modeling, however, is the actual head, torso and limbs of the Footman, and is a lot more complicated.