Your username is your Shopify store name and we we pull this information directly from Shopify for you. You will be able to find this information in your "General Settings" in your Shopify setting.
This username is linked to your stores URL. As we pull this information in from Shopify, you will not be able to change your username for Spently.
If you are having issues logging into Spently, please let us know and we will be able to help you. You can reach us at firstname.lastname@example.org.