Sideloadly or AltStore (note: AltStore requires iOS 12.2 or higher).
For a visual walkthrough on bypassing the update prompt on older devices, watch this guide:
Use a "burner" Apple ID if you are uncomfortable entering your primary credentials into third-party sideloading software. The Web Browser Shortcut (The "No-Install" Fix)
Sideloadly or AltStore (note: AltStore requires iOS 12.2 or higher).
For a visual walkthrough on bypassing the update prompt on older devices, watch this guide:
Use a "burner" Apple ID if you are uncomfortable entering your primary credentials into third-party sideloading software. The Web Browser Shortcut (The "No-Install" Fix)