If you want to change your main character, 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. (News: July 2019)
It's 250 Tibia Coins in the store under extras and then extra services-
July 15 2021 - News: Please note that the vouchers for the Main Character Change which were assigned to accounts in July 2019 will expire and vanish on September 30, 2021, at 23:59:59 CEST. This voucher allows you to change your main character for free once. If you have not used it yet and still want to redeem it, please do so before the expiry date. You can find it under "Products Ready To Use" in your account management. This voucher wasn't able to be used on hidden, scheduled for deletion, or Tournament characters. The main character voucher will appear until the expiration date when you sign in to your account click manage account then the voucher will be listed under products ready to use, then press configure.