CASA data repositories contain our current best knowledge of, e.g., observatory coordinates, leap seconds, ephemerides, calibrator models, etc. Every version of CASA contains the latest files.
Sometimes, however, one may want to manually update the data repository on an existing CASA version. From CASA version 4.4 and onward one can do this in two different ways (independent of the operating system):
1) On the CASA prompt, run "!update-data"
2) on the Unix prompt, run "casa-config --exec update-data" (does not require to start CASA itself)
either command will pull the current data repository from the NRAO database and update it on disk.