If you sent some items from Core to QuickBooks Online by mistake and want to undo the action, you can't do that directly. You need to delete these items in QuickBooks and then remove those record mappings in Core.
When you send information to QuickBooks Online, it creates new records there. Core also maintains a mapping of the Core records with corresponding QuickBooks Online records in QuickBooks Sync Detail > Record Mappings > Mapped Records. If you want to undo the Send action, you should look at the relevant QuickBooks records that appear here (after being created by the sync). You should remove their mappings to Core records so that they appear under Unmapped Records. Check the Core Help for details on that. You should also delete these records on the QuickBooks side so that they disappear from Unmapped Records too. The Send action is now completely undone and you can continue integrating with QuickBooks Online as needed.