You may, or more likely may not, be upgraded automatically which starts 1 calander day before departure meaning 12:01am. You do nothing - they notify you if you make it.

You also can check in on line starting exactly 24 hours before your departure and ask to be out on a seperate upgrade list if 1st class isn't immediately available which almost surely, it won't be. This list has been the sources of numerous posts on how it interacts or doesn't interact with the previous EUA list. But whatever, the story all things being equal, the sooner you check in, the more liklihood you have of getting an upgrade over another silver elite.

There are no free international upgrades to Europe (i assume that's where you're going), only mileage plus cash, and these have to be confirmed a minimum of 72 hours before your flight.