How do I update or change a will I already registered at the DIFC?

You generally can't tweak a registered DIFC will - you replace it. To change beneficiaries, executors or guardians you register a fresh will that revokes the old one; the DIFC charges an administrative amendment/replacement fee (reported around AED 550) plus any drafting cost. Review your will after marriage, divorce, a new child or buying property. (verify current fee)