As the copper is a good conductor, the charge on the capacitor will appear on the large flat surfaces of the copper sheet, with the negative side of the copper facing the positive side of the capacitor. This arrangement can be considered to be two capacitors in series.
If thickness of the copper slab is l, the separation of plates between will be 12(d−l)
The capacitance before insert slab is C=Aϵ0d
The capacitance of each capacitor after insert slab is C′=Aϵ0(d−l)/2
Net capacitance after insert slab is Ceq=C′C′C′+C′=C′2=Aϵ0(d−l)
thus, Ceq>C and if sheet is very thin (d>>l),Ceq=C