This has to be done by the App manager.
To change the name of your intranet:
- From your user profile image, go to Manage > Manage application.
- Under Setup > General, you can specify the name you want to use in Intranet name.
This will change the name globally across the intranet, including what appears in your search bars, browser tabs, emails, etc.
For best practices regarding managing a successful intranet, click here.