Restart your iPhone
Update iOS to the latest version
Delete large attachments in Messages
Remove old iMessage conversations
Clear Safari history and website data
Offload or delete unused apps
Delete and reinstall apps with large caches
Remove downloaded offline content from apps
Delete old mail and empty the trash
Remove downloaded files from the Files app
Clear app-specific caches if available
Back up your iPhone
Erase all content and settings
Restore from backup or set up as new iPhone