[CentOS] best way to upgrade php4 -> php5 on CentOS?

What is the best way to upgrade PHP4 to PHP5 on CentOS? I see from "rpm -qa"
that I'm running PHP-4.3.9.

Should I uninstall this old PHP4 package and then reinstall PHP5?  Or is
there some cool yum upgrade thing I might run?

This is a VMware virtual box for testing, so I'm more concerned about
learning what to do, rather than interrupting anything.
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