Say, Current system:
[root@a1 ~]# uname -a
Linux a1.a1telecoms.co.za 2.6.18-238.12.1.el5 #1 SMP Tue May 31 13:23:01 EDT 2011 i686 athlon i386 GNU/Linux
[root@a1 ~]# cat /etc/redhat-release
CentOS release 5.8 (Final)
MySQL version 5.0.95
Please give detailed steps to upgrade to MySQL 5.5
Centos is built on RedHat whose update paths do not include major version transitions.
Are you looking to transition because of a specific feature you need, or because it is newer?
You could install the newer version and then have to update the elated components, for php, etc.
You have to make sure everything else you have is not affected by changes implemented in the newer version versus what was available in the old.