Right in the beginning of the game tutorial you will be able to choose one of the basic avatars - Garvel or Julia. After that, it will cost you 1M gold to switch to any other one. You will also be able to switch to an avatar previously unlocked using a special hero change item.
In order to unlock an avatar, follow these steps:
1. Open the hero avatars menu, hover your mouse over the one you wish to unlock:
2. Click "Unlock":
3. Use the "Unlock avatar" item (or buy and use it if you don't have one yet) in order to unlock the desired avatar and change your current one:
The avatar that you had before the change will be available as well. In order to switch between the unlocked avatars you need to follow exactly the same steps as before. The only difference is that instead of using an unlock item, you will have to use the change one:
If you have no "Change avatar" items available you can buy them in the store.
Note that switching between hero avatars is only possible once in 20 hours!