How do I change a participant to co owner

Not true at all.

Apple Cards can be shared with family members. Participants can use the card and are supplied with their own cards, but are not responsible for their portion of the bill. You can add a co-owner of the Apple Card or a Participant. A co-owner is also responsible for the bill, and can make his/her own payments from Apple Wallet.

The question the OP has is how to change the status from a Participant to a Co-Owner.

That isn’t supported either. All charges from family members use the family organizer’s payment method. There is no such thing as a co-owner. The only way to accomplish that is for the member to leave the family group.

They asked in the context of Family Sharing, not the Apple Card. Nowhere do they mention the Apple Card, only Family Sharing.

If they meant the Apple Card you are correct, and that speaks well to your mind-reading ability.

Family Sharing can only have one Family Organizer.

You were READING an article on Family Sharing when you posted, so it was a reasonable assumption that you were asking about Family Sharing:

However, epmadsen has answered your question.