Restart your iPhone
Update iOS to the latest version
Delete Safari history and website data
Clear app caches inside individual apps
Offload or delete unused apps
Delete large messages and attachments
Remove downloaded media from streaming apps
Delete offline files from Files app
Remove old mail attachments and mail accounts if needed
Clear downloaded podcasts, music, and videos
Delete old iCloud Drive downloads
Remove outdated backups from iCloud or computer
Reset network settings
Back up your iPhone
Erase all content and settings and restore from backup if System Data remains high