Welcome to the Community. QuickBooks is designed to help you get ahead of your work using the powerful features in the program. I'll be happy to provide the information you need so you can continue to manage your work with peace of mind.
Since QuickBooks Self-Employed and Online are two separate products, you'll have to manually export the mileage from Self-Employed. While there isn't a way to import your mileage into QB Online, you'll also have to manually add it on your QuickBooks Online account. I encourage you to check out this Community post that provides more details and articles with more info about this.
You're more than welcome to ask other questions in the comments below. I'll be here to help.
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