Drop down menu on RR over view screen

Hi Experts

I want to insert a drop down button (Unit of measure) on RR over view screen as show in yellow colour.

The drop down will have several options, based on the selection, the value in UoM field and the Quantity in  Soh should change.

So basically the quantity should be converted based on the Unit of measure selected.

Please do let me how I can find the relevant class method to introduce the new drop down.

thanks for your help in advance.

Regards

Anup