Where to change default messages

Sorry if this is a simple question.
When a site member does not have the right permissions to access a table (after logging in), the following message is displayed:
You don't have permissions to access this table Back to login page
Where can I change this message and others like it? Is it in a file or is there some way of doing it within phprunner 5.0?
If I have to change a file, should I change the files on the server or on the PC? I am afraid that if I change files on the server they will get overwritten during site updates - and if I change them on the PC, then all sites will have the same message. i am trying to customise my sites.
Thanks

Andrew