*Lots of Pics incoming*
Once you change the name to something you want it to be, click save.
It will bring you to this window.
Then, you see a Drop down bar and the name of the file towards the bottom.
if you want to overwrite the original file, don't change the name of the file, just add .truck to the end of it. if you want it the be a separate vehicle, change the name of the file to what you want, just remember to add .truck to the end.
(I know the pics look a little backwards, I went out of the order on steps but that doesn't matter) then, click the drop down bar and click "All Files".
Once it looks like this
Go ahead and save to the desktop, then, drag the new .truck into the .zip of the truck you were changing the name of, that should do it. Hope it helps