When you first install the app, there are chances that your vendors/seller names on your store are not properly set up and you wish to do so before importing your orders in PuppetVendors. Or you realize that a setup/change is required once you have already imported the orders.
Note that when you make a change to your vendor/seller value on your Shopify store, Shopify, unfortunately, doesn't update the value to existing orders. And since Shopify doesn't do that, you will have to manually re-assign them from within the app but only for past orders. New orders will automatically be re-assigned to the right vendor/seller going forward.
Here's a quick walk-through of how you are able to re-assign vendors/sellers on PuppetVendors.