I'm here to provide the steps to ensure you can increase the selling price of stock items in QuickBooks Online (QBO).
We can update the selling price by exporting the product and services changing the pricing and importing it back to QBO. However, since you've mentioned that it'll be 5000 items, you might need to split it into a multiple Excel file since the limitation is only 1000 rows.
Here's how:
Go to Sales.
Choose Products and Services.
Click the Export icon to export the Products and Services to Excel.
Once done, you can edit the pricing to Excel and import it to QBO.
Go to Sales.
Click Products and Services.
Hit the drop-down arrow beside the New tab and choose Import.
Also, you can create and define each specific item price you charge or sell to your customers. For detailed information, I suggest following this article on how to add products and services, track the items you sell, as well as how to edit them: Mange Products and Services List.