Thanks for posting in the Community accounts166,
If you are adjusting a stock list and reimporting you wont be able to import any data which is already within QuickBooks Online as this will cause a duplication, however you are able to upload any new stock items not already recorded in the system.
Please let us know if there is anything else we are able to help with.
QBO offers limited features to manage inventories. Intuit decided to acquire one inventory management app to enhance the basic feature available on Plus. If you declined to have an additional app, consider switching to QBD Premier instead. Deploy a private cloud should you need to access your data by internet connection.