Just couldn't figure out how to do it properly. I've seen the following solutions:
1. adding one of the million possible update scripts using @require
What's the state of art of scripts updating?
For now, I use an @require ( usoCheckup ) which seems to work just fine for all of my scripts here on this site.
Greasemonkey automatically stores the install URL right now, and allows you to check for (and automatically install) updates for your scripts, but there is a draw back: it doesn't automatically check (yet). There is an issue on GM's project page that discusses the reason behind it ( https://github.com/greasemonkey/greasemonkey/is... ).
I'd suggest using an @require for the time being until GM does it natively.
Also, as a side note, Scriptish updates scripts automatically, but of course that only applies to Scriptish users.