Here are simple steps on How to Upgrade from CentOS 5.4 to CentOS 5.5. Open Terminal Window and run below commands

yum clean all

yum update glibc\*

yum update yum\* rpm\* python\*

yum clean all

yum update

reboot

After restart CentOS 5.4 will be upgraded to the latest CentOS 5.5.