To edit GRP files, you'll need RetroGRP. Open WinMPQ and extract a GRP file. For example unit\terran\pheonix.grp.
Open it in RetroGRP. You'll notice it only has 17 frames. Press "Extract Bitmaps", then Extract Unique Bitmaps. Look at the frames in Paint. All the frames point to the right. This is because StarCraft mirrors the frames for the other 180 degrees.
To replace the Wraith, all you'd need to do is put your 17 new frames in the GRP instead of the Wraith's 17 frames. If you wanted a graphic that acted differently however (like a ship with a special animation, or changing the Wraith into a foot soldier), then you'd have to get into iscript editing, which is more advanced.
Since Ojan could do a better job explaining than I can, check out this tutorial:
How to use RetroGRP:
http://www.staredit.net/index.php?tutorial=206