Workflow forms are so customised there is no quick easy fix for this. It will require investigation likely into a view or stored proc in SQL.
Usually there is a flag somewhere on the user or resource record that flags a user as a WFF approver if the approval is driven by a view or stored proc, so you just need to work out what that is.
Open up Workflow Form Designer
Edit the form in question
On the Approval tab
Edit the stage of approval that needs the users changing
Identify where the list of users comes from:
If it is from a set of specific users, add and remove users as required
If it is from a stored procedure or view - You will need to engage with an Access Consultant who can review look up the stored procedure or view in SQL, identify how the users are flagged as approvers and provide the guidance