Store Logo
In the Admin panel -> go to Storefront -> Logo -> Select "Upload a custom image to use as your logo"

Also, here is an article of BigCommerce for your reference: https://support.bigcommerce.com/articles/Public/Uploading-a-Custom-Logo/
If you want to use text to make your logo:
In the Admin panel -> go to Storefront -> Logo -> Select "Enter text to display as your logo"

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