Private Sub cmdsave_Click()
Set rs248 = New ADODB.Recordset
rs248.Open "Select * from tblStock", cnn, adOpenDynamic, adLockPessimistic
With rs248
If Not .EOF Then
rs248.Fields("category").Value = catecmb.Text
rs248.Fields("Subcategory").Value = subcatcmb.Text
rs248.Fields("Mused").Value = metalcmb.Text
rs248.Fields("Weight").Value = weightcmb.Text
rs248.Fields("Stin").Value = rs248.Fields("Stin").Value + quantitytxt.Text
rs248.Update
rs248.Close
Else
rs248.AddNew
rs248.Fields("Category").Value = catecmb.Text
rs248.Fields("Subcategory").Value = subcatcmb.Text
rs248.Fields("Mused").Value = metalcmb.Text
rs248.Fields("Weight").Value = weightcmb.Text
rs248.Fields("Stin").Value = quantitytxt.Text
rs248.Update
rs248.Close
End If
End With
end sub

i enter
necklace pendent gold24k 3.2 8
necklace pendent gold24k 3.2 10
ring xyzz gold22k 1.1 15

table should store
necklace pendent gold24k 3.2 18
ring xyzz gold22k 1.1 15