If you need to update quantities on all combinations of multiple products by the same stock value, you can use the mass-edit option in Store Commander, located in the popup menu on products in the interface.
First select the products you want to update combinations onto, to then right click to display the popup menu and select Mass-edit > Combinations > Quantity
In the window, enter the stock value to add - note that you can also remove stock - and validate on OK
And that's done!
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