This is explained in the manual, on the "Live Update" section.
There are two types of updates, minor and major. The minor updates are listed as "Couatl Live Update" and don't require doing anything, and can be applied even while FSX is running.
The major updates are listed as "Addon Manager update", and require to exit from FSX and launch either the Stand-Alone Addon Manager installer OR the GSX installer you already have, no need to download them again.
In case you'll need a whole new installer, we'll announce it as a new full release, possibly indicating on the release notes you'll have to download the complete GSX installer again, right now we haven't done anything that needs this.