Here are the ways you can try:
1. Shut down your iPhone. Wait a few minutes before turning it back on. Doing so will free up system resources and clear temporary files from your storage. It’s a lot easier than a reset of your phone.
2. Clear cache in Safari. If Safari is your browser of choice, the cache can take up significant storage. Clear it by going to Settings > Safari > Clear History and Website Data
3. Delete offline content. Periodically delete offline content from your apps—such as downloaded shows in the Netflix app. You need to do this individually in the apps. Where possible, set them to auto-delete.
4. Disable features. Apple devices and apps come with a lot of features, but you probably don’t need all of them. Disabling the ones you don’t use could help prevent the accumulation of system data. To disable features, click through your device’s settings menu and toggle off the ones you don’t use. Examples include the Translate app’s On-Device Mode (translates languages when you’re offline) and Apple Maps’ feature that knows where you parked.
5. Identify rogue apps. As mentioned above, try temporarily eliminating apps from your phone to determine if there’s one using an inordinate amount of system data storage. It’s detective work but it can save you hassle in the long run.
6. Factory reset. A factory reset followed by restoring your backup files is the surefire way to lower your system data. To do so, head over to Settings > General > Transfer or Reset iPhone > Erase All Content and Settings. If you do that, know that you will also be signed out of your Apple ID and all your apps, media, and personal data will be removed.