You will find most (if not all) text that is visible to the user in one of the lang files. Assuming you are using english, that will be lang.en. If you only want to change what the user sees that should be all you need to change. The option is under "// Used in edit_entry.php".
If you start changing the sql field names, then you will have to make many changes in most of the php files, which will be a pain when you choose to upgrade to a new version. The MRBS mailing list is probably the best place to ask that sort of question. You will need to change at least day.php, month.php, week.php, edit_entry.php, edit_entry_handler.php, report.php and probably more.
I would strongly recommend only making custom changes to the code if absolutely necessary. Changes to lang files are much easier to redo come the new version.