A full FAQ on pricing is available here: https://help.github.com/en/github/getting-started-with-githu...
Hope that's helpful!
"Your account can not be downgraded yet because one or more of your private repositories is over the collaborator limit for the free plan. Please make sure that each of the private repositories owned by your account below has 3 or fewer collaborators before downgrading your account. Questions? Please contact support@github.com."
Am I missing something or is this not implemented yet?