Hello. Are you trying to install the update directly on your iPhone? If so, then you may want to try deleting and downloading the update again. You can do this by following the instructions below.
- Go to Settings > General > [Device name] Storage.
- Find the update in the list of apps.
- Tap the update, then tap Delete Update.
- Go to Settings > General > Software Update and download the latest update.
If you continue to experience problems after downloading the update a second time, then you may want to try updating your iPhone by connecting it to a computer. You can do this by following the instructions here for macOS Catalina or later and here for macOS Mojave and earlier and Windows PCs.