In terms of syntax then you do..
StrSQL = "update [tblEst] set [PrCostQty1] = " & [prCostQty1] & " , [PrCostQty2] = " & [prCostQty2] & " where [EstNum] = " & [EstNum]
However this statement makes no sense to me as you appear to be trying to set something to itself.
Or is the [prCostQty1] within the quotes somehow different to the [prCostQty1] that is outside the quotes?