How do I update the SKUs and GTINs that imported incorrectly from Shopify

So I have the plugins for Brands and UPC/GTIN/barcodes also. Both of which are vital to selling on marketplaces. Your tool used barcodes for the variant SKU and didn’t add the SKU at all.
Also, all of the variants are multiple choice. For example; a size “Large” has a barcode in place of the SKU of 192556852325. Well, there is a dropdown that allows me to select any size for the same SKU so no matter what variation I select the SKU is always 192556852325. All variants are showing as Parent.
Thanks so much for your help!