Go to the zip and look for the .png with the name of the truck, for example: Bigfoot.png, open the png file with any editor (like paint, paint.net, photoshop or gimp) and start editing the truck.
If you want to upload it here you will have to clone the truck (if you dont clone it, the truck will overwrite the parts that you modified). Its not that hard, there is a tutorial in the tutorial section. If the truck does not overwrite anything the admins may approve it
I hope that you understood lol