Restart your iPhone
Update iOS to the latest version
Clear Safari history and website data
Delete large message attachments
Remove downloaded offline content from apps
Offload or delete unused apps
Delete old mail and attachments
Remove downloaded podcasts, music, and videos
Clear app caches inside individual apps
Delete and reinstall apps with large cache
Check and remove old iPhone backups
Turn off and back on iCloud sync for large apps
Reset network settings
Back up your iPhone
Erase all content and settings
Restore your iPhone from the backup
Set up the iPhone as new if System Data remains high