Upgrading at the airport

Hi,
I hope someone can provide a little guidance.
I'm flying LGW-LAS this week on an "X" fare which is not eligible for upgrade with miles. (there are 6 PE seats for reward available on this service)
I called VS to see if there was a way around this, and was advised that I can use miles to upgrade at the airport.
My questions are:
How early should I try to be at the airport to get the upgrade Who do I ask - the check in agent or ticket desk What payment methods are accepted for the associated taxes Can I upgrade the return leg at Gatwick or would I have to request this at LAS
Thanks in advance!
I hope someone can provide a little guidance.
I'm flying LGW-LAS this week on an "X" fare which is not eligible for upgrade with miles. (there are 6 PE seats for reward available on this service)
I called VS to see if there was a way around this, and was advised that I can use miles to upgrade at the airport.
My questions are:
How early should I try to be at the airport to get the upgrade Who do I ask - the check in agent or ticket desk What payment methods are accepted for the associated taxes Can I upgrade the return leg at Gatwick or would I have to request this at LAS
Thanks in advance!