You can reference this page:
https://www.aircanada.com/ca/en/aco/...ade-offer.html
"For a limited time, take advantage of our exclusive eUpgrade promotion^ on eligible comfort and Flex fares, which includes the waiver of Add-ons for Flex fares. Plus, don’t miss the opportunity to request an upgrade from Standard (previously Tango) fares as well as Aeroplan Fixed Mileage Flight Rewards™. These limited-time offers are valid for eUpgrade requests made between September 20, 2018 and January 10, 2019, for travel within Canada and between Canada and the U.S."

Problem is if you then go to the booking window page:
https://www.aircanada.com/ca/en/aco/...ng-window.html
It says you can only request your upgrade for flights in Canada/US at check-in.

Bottom line - if travel is before the 10th, you can do it at check-in. If after the 10th, you'll need to hope they renew this "limited time offer" which has been renewed regularly.

This is Assuming you booked fixed mileage and not "market" rewards.

Since Puerto Vallarta isn't in Canada and the US you will probably be out of luck for that unless they modify the rules.