Having an active email account @polito.it or @studenti.polito.it, you are entitled to access the services provided by the Office 365 Education E1 plan (see https://products.office.com/en-us/academic/compare-office-365-education-plans for further details).
In particular, you can download and install the last version of the Office 365 ProPlus suite (in all the supported languages, for PC and Mac, 32 and 64 bit versions). Your copy of Office will be already activated, no further action is required on your part
You have to first sign up with your @polito.it or @studenti.polito.it mail account and complete the registration (following the instructions you will receive via email). You will then be able to log in with your new set of credentials (@polito.it or @studenti.polito.it mail account + password chosen in the registration process)
See further details at http://www.areait.polito.it/servizi/default.asp?outline_level=3&id_progetto_servizio=344&outline=1%2E47%2E15 (content in Italian)