Only the Clan leader or a co-leader are able to manage Clan members. To do so, navigate to your clan's page, then tap Clan Members. Then, you can browse through your Clan members and tap the player profile of the member you wish to promote, demote, or kick. You will see buttons on the player profile for the options that are available to you.
As a leader, you can promote members to co-leader, or demote co-leaders back to member. Clan members can always be kicked, however to kick a co-leader, the co-leader must first be demoted to member by the Clan leader before the option to kick is given.
A co-leader can promote other members to co-leader, but will be unable to demote fellow co-leaders. Co-leaders can also kick Clan members (but not other co-leaders).