If your Prismic repository is on the Free plan, it is super easy to transfer ownership to another person.
Open your repository and navigate to the Settings / Users. From here you should be able to enter the email address of the person you want to transfer ownership to.
When you click on "Transfer ownership," that person will receive an email to confirm the ownership.
Until they accept ownership of the repository, you can cancel the pending change of ownership request by clicking on the "Cancel" button.