Where to get the thing to change my class?

You get a chance to change your class as you level up every 3 levels. You can also buy the change class item as part of one of the packages on offer for in game purchases.

You can usually find the package in the list of options by tapping one of the offer icons that show up on the top right hand side of the main screen. It may look like a truck flashing lights or have some other image depending on the latest offer. Swipe through the different options to find one with the change class item.