In order to transfer ownership of a Vercel Team to another Vercel user, you will need to add the other user with the role Owner on the Vercel team. Once the newly promoted user is successfully an Owner on the team, that user can freely remove the original Owner or demote the original Owner to a Member.
If you have any further questions about transferring ownership of a Vercel team, please contact Vercel Support using the support form available from the Vercel dashboard.