The best way to update or modify existing sku's will depend on whether or not the original sku will continue to be found in future orders and whether or not you need to keep your older scanned order records in GroovePacker.
If you are making a clean break from the old sku (ie. all future orders will reference the new sku) AND it's not a problem to delete the orders you've already scanned in GroovePacker. Then the absolute fastest and cleanest way to make the change is to:
1- Delete the existing orders* in GroovePacker (unscanned orders can be deleted as well and then re-imported after the change is complete)
2- Remove your existing products from GroovePacker.
3- Import a product csv file that has the new SKU. This may be exported from another app, like Shopify, or you can export your current GroovePacker products to CSV, then modify/update the sku before re-importing them.
4- Import your orders that reference the new sku and you should be all set.
*If you need to maintain your previously scanned orders you can leave out the step of deleting the orders but the product removal time is greatly increased due to the edits required on orders containing the items. It can take hours if there are many orders.
On the other hand, if the change to the new sku will be gradual, it will be better to keep your old SKU associated with the product and add the new SKU as an alias. The advantage is that either SKU can be present in incoming orders and it will be processed correctly. This is done by importing a CSV with at least two columns. One is existing SKU, which during import is usually mapped to "SKU". The other is the new SKU usually mapped to SKU2.
Info on importing Alias SKUs can be found here. Taking a product backup prior to importing the new SKU is recommended since the rest of the product data is not being removed.
If you are uneasy about making the change you can email your CSV file, including the two SKU columns, to support and we will import it for you. Please clarify your intention to have the new SKU added to your existing SKU in the message.