Well, you need to add repository that include php 5.5 version. For example, use our colleague's repository -
http://centosup.ispsystem.info
When repository be added just replace php
# yum install yum-plugin-replace
# yum replace php-common --replace-with=php55-common
After this put "path php_ver 5.5" to ispmgr.conf and restart control panel with command
/usr/local/mgr5/sbin/mgrctl -m ispmgr exit