Hi Francesca
Do you have access to the program files on the web server that Totara is installed on?
If you do then should find the file in the folder Totara is installed in which is usually mod\certificate\lang\en - or search for the file on the file server as the folder structure is different in later versions of Totara.
If you don't have access you will need to ask the server admin to access these.
Regards