How to add cost sub element material overhead to an existing item with material cost value

Hello

We need to add cost sub-element material overhead to existing purchasing items.

I add setup on new sub-element by item category with total value rate.

I want to update this new material overhead for all existing purchase items by using the open interface tables.

I know that the on hand value amount will be affected after this update.

What are the steps that I need to be consider and which of the open interface in need to use?

I will appreciated for any help

Regards

Benny