How to change the appearance of a character

I’m attempting to make a simple neo-futuristic game level, with a few non-hostile NPCs and whatnot. The problem is, the NPCs I am using are just standard 3rd person mannequins, which in a game look terrible. I searched on the web for a while, trying to figure out how to change the appearance of these mannequins, and came across a lot of stuff about meshes that I didn’t understand very well. So this leads to my question.

Is there a way to use UE4 or a 3D modeling software to make custom image characters? To help clarify, I’m just trying to change the appearance of a mannequin or actor class, nothing more. So is this possible? Please help.

The Default Mannequin you see at the start is just the default skeletal mesh. If you wanted you own character you would need a modeling software.