To stack tabs above and below in Notepad++, follow these steps:

1. Open Notepad++.
2. Navigate to the top menu and click on Settings.
3. From the dropdown, select Preferences.
4. In the Preferences window, click on General from the left sidebar.
5. Look for the Tab Bar group within the General settings.
6. Check the option labeled Multi-line.

This will enable the multi-line tab bar, allowing tabs to stack above and below, which can be especially helpful when working with many open files.