I am loading data from a staging table to a cube

The key columns in my staging table are the dimensions in the cube and
the non-key values in the staging table are the measures in the cube.

All I want is if the record in the staging table matches the current
value in the cube I want the entry in the cube to be overwritten

Key columns
===========
Month
Material
Year
Customer
Store

Non-Key Columns
===============
SalesQty
Sales$


Can any of you kindly share how this can be done most efficiently as I
would have more than 2 million records in the staging table and about
3 mill records in the cube at any point of time.

Thanks
Karen