Applications will be available at the end of March, 2016 on Minerva for students wishing to transfer into the Faculty of Science or to transfer into the B.A. & Sc. program for Fall 2016. The application deadline on Minerva is May 15, 2016 for Fall 2016 term.
If you are already a McGill student pursuing an undergraduate (Bachelor) degree, and you would like to change your degree, there are two ways to do this - Inter-faculty transfer, and Intra-faculty transfer.