Account Upgrade Credit is the currency of Account Upgrades. You can get them from the Monthly Checklist,

You can spend AU Credit at the Account Upgrades

Goals, Temples, some Events, Slate Pyramid (without Kamilah Pyramid Treasure Map), Collections, and by paying for them with REAL MONEY with PayPal.

You can also win £10 AU credit by playing Bingo and winning a full house on your Lucky Star number.