If you want to change existing units, then change the data - specifically the "icon" fields at the bottom of the unit page. One each for green, winter and desert if needed.
Stick in your new icon numbers and hit F12 - job done.
Of course, you have now altered your OOB and so it will now fail PBEM. But your cut and paste to new unit idea also invalidates the OOB checksum.
In Mobhack Help -> Help -> Units Tab Paragraph beginning
Icon ...
IF however you mean "How can I change the SHP file for unit X" - you are into a completely new direction. Search the forums on SHP file editing and SHPEd, game palettes etc.
A quick search on SHPEd gives this thread -
http://forum.shrapnelgames.com/showt...ighlight=shped
Most things have already likely been discussed - so use the "advanced search" forum utility to look for useful keywords in
BOTH forums (WW2 and MBT) for that sort of thing.
cheers
Andy