how can i reduce the system data?

If the system data is not corrupt you can’t reduce it; that’s data that the system needs. If it seems unusually large you can back up the phone, restore it to factory settings, then restore the backup.

1. How to back up your iPhone, iPad, and iPod touch - Apple Support
2. Restore your iPhone, iPad, or iPod to factory settings - Apple Support - follow the instructions to restore using your computer, do not using Settings/General/Reset (which is not included in those instructions)
3. Restore your iPhone, iPad, or iPod touch from a backup - Apple Support