Updating OSM data in a PostGIS database?

Hi, I’m pretty new to using OSM data in a PostgreSQL database. I’m curious about how to update it. Do I run the osm2pgsql command just like I did the first time? Will it overwrite old data? Merge the two? Thank you for any help.

Maybe you can find some hints at http://wiki.openstreetmap.org/wiki/Minutely_Mapnik ?