It has many public use airports in its database (I am not sure what the
criteria is).

(Pardon me while I blow my own horn)
If it's supported by EasyGPS (http://www.easygps.com/) or GPSBabel
(http://www.gpsbabel.org/)[1], then you can generate a better database of
airports at my site http://navaid.com/GPX/ and load it into your GPS.

[1] gpsbabel -h shows "magellanx - Magellan SD files (as for eXplorist)"
as an option.

Unfortunately, the Magellan Explorist 300 has no provisions for updating
firmware or software.