Sopon,
As Louis mentioned, customizing USS is always a challenge and remind that by doing so, if you upgrade it then these customization might need to be re-done.
Anyway, one thing you could try is running the "Agent Ransack" utility to search for the string / message you want to modify.
Ex.:
In this case, it seems these messages are stored in language files under "C:\Program Files\CA\Self Service\OSOP\tomcat-7.0.40\temp\0-front-office-hook\WEB-INF\classes\content".
Check if that would help you.
Regards,
Roberto