Two days prior means 2 calendar days.

So if your flight is on friday, then you can upgrade on wednesday. It becomes wednesday, at midnight tuesday night/wednesday morning.

So at 12:00:01am wed you could upgrade a flight anytime on the following friday.

BTW: The time zone of the departure city is used.

Butcher: it's not that friday doesnt count, but 2 days prior to friday is in fact wed.