You can use Skus (Tab)->Import/Export->Export SKUs. Edit in Excel. Delete all columns except the SKU and the attribute. and use the “Import Attribute” menu to import back.
Another way is to enable switch 102 in the store settings. Change the attribute value for one of the SKUs. Hold CTRL key and click Save. The system will ask you to select SKUs to apply the change you have just made.
You can also email a detailed request to us. We will change it for you.