Did you try the bulk edit for this yet? If not, it could be the best way forward for you on this. I was thinking if it could be the metric system aspect of the store which you can change in settings - Settings > General > Standards and formats you’ll need to change the Unit System and the Default weight unit.
But after looking at your store, it seems like this is in the product aspect and variants rather than the settings. You can bulk edit the variant’s for your products with the following steps or by checking the help guide here:
From your Shopify admin, go to Products > All products.
Click the name of the product that has the variant that you want to edit.
In the Variants section of the product details page, click Edit beside the name of the variant that you want to edit.