To give another account the rank of Clan Owner, the account you want to move to Clan Owner should be ranked Deputy Owner, and all other Deputies should be temporarily demoted to Overseer as a precaution. This ensures that the Clan Owner rank isn't mistakenly given to someone else.
While only one clan member is ranked Deputy Owner, as the Clan Owner, demote yourself to Admin. This will cause the vacant Clan Owner spot to be filled by the Deputy Owner. The new Clan Owner can then fix any ranks of clan members as necessary.