An exit seat - they can't be pre-assigned on Emirates, best bet is to get to the airport early to check in, I normally get there around 3 hours before my flight, I have a very good success rate.

Purchasing an upgrade, log onto the Emirates web site, go to Manage a Booking, key in your Booking Ref and last name. Down the right hand side they'll be a button saying Purchase an Upgrade - click that. Be prepared that it might say something along the lines of "this ticket was purchased through an agent please contact them directly".