A member in a community always has a role as either reader, curator, manager or owner. See about communities for details on roles and their permissions.
Only community managers and owners can change roles of other members, and only owners can change roles to an owner.
Go to the Members-tab of your community.
Find the member you wish to change the role.
Change the role of the member using the drop-down menu in the Role-column. Note that only community owners/managers can change roles, and that you cannot change your own role, nor as a manager change a member's role to an owner.