If you use Shopify and have the Printify Shipping Calculator enabled, you don't need to do anything - the shipping rates are updated automatically in case of any changes to the rates.
However, if you're using a flat rate setup for your Shopify store, you'll need to update the rates manually by following these steps:
From your Shopify admin page, go to "Settings" → "Shipping and delivery".
Next to the shipping profile that you want to edit, click "Manage rates".
Next to the rate that you want to edit, click the "..." button to open additional options.
In the drop-down menu, click "Edit rate".
Make any changes, and then click Done.