B6 does not do upgrades; if you want to sit in Mint, you'll have to pay for it.
They do offer at the airport only on the day of your flight a flat upgrade to Mint for $499. Doesn't matter what price you paid for economy.
But it can only be done at the airport, not over the phone/web, like the VX 24 hour upgrade policy.
I have done it twice. Once I asked the counter agent during checkin and got it for the $499. The other time, the gate agent was announcing the upgrade option and was actively trying to sell it and I got it for the $499.