Hi
There is no standard way to only update the costs of the modified items
You can however update all and in the process the new costs will be changed, this is a rough idea of how it works, if you need more detailed info let me know and i will go and dig up the exact method
What you need to do is to use the "Standard Cost Worksheet", add the raw materials to the sheet, add the finished goods and then roll up the standard costs
This will take the cost of the raw materials, add the cost of overheads and produce a finished goods price
The standard cost worksheet then has an option to implement standard cost to update the item cards for you
Hope this helps
Nev