The company must consent to the transfer; so the seller should contact the company for transfer mechanics.
Here is the current safe transfer provision and it generally says that both the company and the holder must consent to the transfer unless the seller is merely transferring to an affiliate of the seller:
“Neither this Safe nor the rights in this Safe are transferable or assignable, by operation of law or otherwise, by either party without the prior written consent of the other; provided, however, that this Safe and/or its rights may be assigned without the Company’s consent by the Investor to any other entity who directly or indirectly, controls, is controlled by or is under common control with the Investor, including, without limitation, any general partner, managing member, officer or director of the Investor, or any venture capital fund now or hereafter existing which is controlled by one or more general partners or managing members of, or shares the same management company with, the Investor; and provided, further, that the Company may assign this Safe in whole, without the consent of the Investor, in connection with a reincorporation to change the Company’s domicile.”
If the company consents, they decide how they want to handle the transfer. It could be one three way agreement or two, two way agreements.