What OS platform are you on? On Debian you can simply apt-get upgrade (assuming you have correct sources). On FreeBSD you can portupgrade databases/postgresql72. On RedHat/Mandrake you can probably use one of many installer/urpmi/whatever package managers.
If you were to do a source install -- this again is not a problem. Check INSTALL and README files packaged with the distribution tarball. You need to remember just one thing: stop the server before you start the install process.
7.2.3 -> .4 should not be a problem (you don't even need to pg_dump/pg_restore the database, though you may still want to do it if you wish). Moving to 7.3 is more chalenging -- you will have to dump/restore the db.
