Cross your fingers and hope.
If you ask for an upgrade, you will may be offered one, for a fee. Free upgrades seem to come to those who are just plain lucky. There doesn't seem to be any rhyme or reason to getting one. But asking doesn't seem to be the ticket.
Some people believe early booking will help, but others have been upgraded with late bookings while earlier folks have gone un-upgraded. Booking a category rather than a room is another technique to improve your chances, but there are no guarentees.
Book the room you are happy staying in. If we could get free upgrades for the asking, why would anyone pay for a cat 6?