Restart the iPhone
Update iOS to the latest version
Delete large message attachments
Clear Safari history and website data
Remove downloaded offline content from apps
Delete old podcasts, music, and videos
Offload or delete unused apps
Clear app caches inside apps that allow it
Delete and reinstall apps with large cache usage
Remove old iCloud Drive downloads stored on device
Turn off and back on iCloud Photos if needed
Back up the iPhone
Erase all content and settings
Restore the iPhone from the backup