Assuming column A is productid, column B is variantid and column c is weight, then you need to use formulae to create a sql statement in column d of each row. Also, assuming the first row is headers then your data starts on the second row. So in column d your formula would be
Code:
="UPDATE ProductVariant SET Weight=" & C2 & " WHERE VariantID=" & B2
Weight must always be numeric allowing decimal places. Copy/drag it all down. Then copy and paste your SQL statements into SSMS and run.
Run it in a dev environment first to see that you get what you expect.