You should be able to modify the default approvers for the Kemin Aptivax - Kemin Biologics PO sign-off application 

via this URL: Digital Sign-off default approvers.

Both fields need to be changed - Approver and SignUserEmail.

This change will not affect the already running approval flows (if there are any).

Click here (for the developers) to check for running "(MG) KAE Aptivax-Biologics (PO) sign-off - CHILD" workflows