is it possible to down grade the os

Hi,

I run a small holiday rentals business and use a mac air running OS X 10.7.5

I recently purchased another mac air for someone to help out running the business but the calendar drop down menu on the email enquiries isn't the same as mine on the 10.7.5 (the newer version is less efficient for this purpose). The 11 inch mac runs on 10.8.3.

Is it possible to get the old calendar back? I'm happy to replace the 10.8.3 for the 10.7.5 if I need to but how would I do it?

Thanks!