You will have to pay 250 for Main Character Change. See Faq:
Can I trade my main char?
Yes. In this case, no other character is designated as main character automatically, however. If you would like to have a main character again, you have to buy a "Main Character Change" in the Store.1
However, as per TibiaWiki(Fandom) "Moreover, if you create a new character, you can also set it as main character for your account."2
1. https://www.tibia.com/news/?subtopic=newsarchive&id=5692&postid=39013848#faq9#
2. https://tibia.fandom.com/wiki/Main_Character
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