how to assign members to different levels?

Hello experts,
I created 3 levels of membership: free, member and premium.
Also I created 3 users and named them: free, member and premium respectively.

But I can't assign users to each of the 3 levels of membership I created, because the number of free/member/premium is 0.

Can you please advise whether can assign users to each of the 3 levels of membership I created?

Thank you