You can upgrade your mysql version from 5.0 to 5.5 by using following steps

    1. Enable remi repository for yum

wget http://dl.fedoraproject.org/pub/epel/5/i386/epel-release-5-4.noarch.rpm
wget http://rpms.famillecollet.com/enterprise/remi-release-5.rpm
sudo rpm -Uvh remi-release-5*.rpm epel-release-5*.rpm

2. Run mysqldump to export data out from MySQL 5.0

mysqldump -uroot -p –all-databases > mysqlbackup.sql

3. Run ‘yum –enablerepo=remi install mysql mysql-server mysqlclient’

4. Run mysql_upgrade (part of mysql-server.rpm) to clean up the upgraded package

mysql_upgrade -u root -p

5. Adjust my.cnf (version 5.0) for version 5.5

6. Restart MySQL if needed