What do you mean? Change gun gun skin or position?

You cannot change the skin without 'hacks' like what you can often find on sites like Gamebanana, which has custom skins you put into your folders and overwrite your default files. Usually not a good idea, but not lua hacks so it could be legit. I play tf2 tournaments and once i had a mod to change pyro's firemans hat to a space marine suit from warhammer 20k, it just went missing texture but still held shape. In general, I hope you mean gun position, like LunaR probably does.

You can't change the weapons' skin unless you have it in your inventory.

I think he's talking about creating skins like the ones in the workshop.