Asset Server? Well, that pile of ridiculously overpriced shit is your first problem right there.

Using the
free and
much more powerful Git or even SVN you'd have much easier time.
Straight from NGUI's readme:
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How To Update NGUI
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If you have the Professional Edition of NGUI that comes with Git access, just pull the latest changes.
If you have a Standard Edition:
1. In Unity, File -> New Scene
2. Delete the NGUI folder from the Project View.
3. Import NGUI from the updated Unity Package.