My name's Alex - I'm a Guru here with Shopify.
The easiest way to do this would exporting a Product CSV and changing Variant Inventory Tracker. Delete the data for the whole column apart from the title. A blank cell indicates that you do not want items to be tracked, otherwise you'd enter the name of the app tracking inventory (ex. 'shopify', 'shipwire').
You can export a CSV file by heading to your Products menu and clicking "Export" at the top, then you can Import it with it's sister button when you're ready.
Let me know if you have any more questions!
Have a good day!