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We present a method of changing the effective spin-spin interactions of electrons localized in quantum dot molecules from antiferromagnetic to ferromagnetic using electrical means. We show that starting with antiferromagnetic interaction of two electrons localized in two quantum dots, the effective spin-spin interaction can change to ferromagnetic by allowing interaction with a third dot containing two spin singlet electrons. The total spin of this four-electron complex can be tuned by changing the third dot potential from singlet to triplet. This is demonstrated using the linear combination of harmonic orbitals combined with the configuration-interaction method mapped onto the Hubbard and an effective model for the triple dot system. All three approaches predict singlet-triplet spin transition, i.e., a mechanism for changing the magnitude and sign of the effective Heisenberg interaction between two localized spins.