you can follow these steps:

1. Enable Discounts: First, ensure that the "Discount" feature is enabled in your Odoo configuration. Go to the "Invoicing" module, click on "Configuration" in the sidebar, and check that the "Discount" option is enabled.

2. Create or Open an Invoice: Navigate to the "Invoicing" module and open the invoice for which you want to apply a global discount.

3. Add a Discount Line: Inside the invoice, scroll down to the invoice lines section. Click on the "Add a line" button to add a new line. In the new line, enter a negative quantity (-1) and set the unit price to represent the discount amount. For example, if you want to apply a 10% discount on the total invoice amount, enter a quantity of -1 and a unit price of -10% or -0.1 (depending on the currency format).

4. Save the Invoice: After adding the discount line, save the invoice.

5. Check the Total: The invoice total will automatically adjust to reflect the global discount. You can verify the discounted amount in the "Total" field of the invoice.

6. Validate the Invoice: Once you have confirmed the discounted amount is correct, you can proceed to validate the invoice. Click on the "Validate" button to mark the invoice as validated.

By adding a negative quantity line with a unit price representing the discount amount, you effectively apply a global discount to the invoice. Odoo will calculate the discounted total based on the discount line you entered.

Hi,

If you need to do it without any third party app/custom code, you can add a line with description as discount and with negative value and provide discount.

If you don't need like this, you may have to add custom code to make it possible or check for the available third party app's in the store:  https://apps.odoo.com/apps/modules/browse?search=global+discount

Thanks