Seb,
The ability to change a project leader is only available on our side, for security reasons. But, it's a pretty easy process. First, make sure the person you want to be the new leader is a member of the project. Then, use the Contact Sales button to send in a mail to the DevRel team here, listing out the Project ID and the name of the person you want to be the new leader.
We'll take care of it, and you'll be all set.
Hope that helps!