How to change RFx Response which Status already Accepted and Approved

Dear SRM Expert,

I have 1 RFx which already has RFx Response. There are 3 line items in this RFx and buyer should tick Accept for all these line item during Accepting RFx response. Unfortunately he don't. The RFx response has then be approved and PO already created for the 2 line items. How can we change the RFx response to Accept for the 1 line item left?

Appreciate your advise as I'm still newbie in SRM field.

Thanks you.