- Go to the org settings.
- Select GitHub Apps.
- Find the app and select its "Configure".
- In the title choose "App settings".
- Open the tab "Install App".
- On the installation of interest, select the settings (gear icon).
The installation ID is the trailing number in the resulting URL:
https://github.com/organizations/<orgname>/settings/installations/123456789
installation ID --^^^^^^^^^^