I had the opportunity to work on a Red Hat Enterprise Linux box that was running version 3 of the software – but I’ve been told this should work on 3 or 4 (and possibly 2, but who cares about 2 anymore?). Starting with 4 and perhaps 5 through 6 you can use:
yum update

But before these good days, you had to use:
up2date -l
Lists all the available updates
up2date -u
Download and update all available packages (not kernels)
up2date -uf
Download and update all available packages
up2date PACKAGENAME
Update specific packages (up2date php for example)