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Hyde here from Shopify. Interesting question!
The weights and formats used in your store are set in Settings > General, as you can see in my own test store:
So you will need to ensure that this is set to the metric you want to be used when importing products.
You can update products in bulk using the bulk editor (more here) by going to Products > Select All > Edit Products:
Using a CSV is a great alternative means of updating your products in one go, but you need to be sure to use the correct CSV format to ensure there are no errors during upload. More on that here.
I hope this helps and if you have any other questions I'm happy to hear them.
All the best, Hyde.