An IPSW (iOS Software) file is a firmware file used by Apple devices. It contains the operating system and other software components needed to restore or update iOS devices like iPhones, iPads, and iPod touches.

You can download unsigned IPSW files from various trusted websites that archive these files. However, be cautious and ensure you're downloading from a reputable source to avoid security risks.

To install an IPSW file on an iPhone:
1. Connect your iPhone to a computer with iTunes or Finder (macOS Catalina and later).
2. Open iTunes or Finder and select your device.
3. Hold the Shift key (Windows) or Option key (Mac) and click on 'Check for Update' or 'Restore iPhone.'
4. Select the IPSW file you want to install.
5. Follow the on-screen instructions to complete the process.

Installing an unsigned IPSW file is generally not possible on modern iOS devices due to Apple's security measures. Unsigned IPSW files are typically older versions of iOS that Apple no longer signs (authorizes for installation).

Unfortunately, it's not possible to restore an unsigned IPSW without SHSH blobs on modern iOS devices. SHSH blobs are unique signatures required for downgrading or restoring to a specific iOS version, and they need to be saved while Apple is still signing that version.