Its easy just open the folder where you downloaded it, launch the terminal, and enter this command:
sudo dpkg -i stable_123.0.5669.23_amd64.deb
Using the terminal lets you install the package directly, and if anything goes wrong, the terminal will show you error messages.
I prefer terminal over installing it through software app, terminal is more friendly for me !!
If you have any more .deb files just use : sudo dpkg -i 'package-name'