Hi,

If you know the external id, then you can use it to inherit and using xpath, make necessary modifications.
If we go to Settings -> Technical -> Reports, we can see all the reports. From there, select the report you want to customise and check the external id.
You can refer to this blog for more information. Even though it's in version 14, you can follow the same steps for 16.

https://www.cybrosys.com/blog/inheriting-existing-pdf-reports-in-odoo14

Hope it helps

In other words, how can we inherit and modify existing pdf reports