"If you want to change your main character you can do this for free once by using the Main Character Change voucher that has been assigned to your account. You can use this voucher in your account management page to choose a new main character. Please note that you cannot select characters that are hidden, scheduled for deletion, or Tournament characters.
If you want to change your main character again, you can purchase a Main Character Change in the Store. Moreover, if you create a new character, you can also set it as the main character for your account."
To find this voucher in the store it will be under extras and then extra services.
To find the free voucher it will be on the website when you sign in to your account click manage account then the voucher will be listed under products ready to use, then press configure.