Back up your iPhone using iCloud or your computer
Open Settings
Tap General
Tap Transfer or Reset iPhone
Tap Erase All Content and Settings
Enter your passcode if prompted
Confirm the erase action
Wait for the iPhone to restart and complete setup
Set up the iPhone again from a backup or as new
If you only want to reduce storage without erasing: open Settings
Tap General
Tap iPhone Storage
Review the list and remove large items (e.g., apps, videos, downloads)
Offload unused apps: tap Offload App (or Delete App for full removal)
Delete large message attachments: open Messages, open a conversation, delete photos/videos, or remove entire conversations
Clear Safari data: open Settings, tap Safari, tap Clear History and Website Data
Clear app cache (if available in the app settings)
Remove downloaded media in streaming apps (offline downloads)
Disable or remove iCloud Drive files you no longer need: open Settings, tap your Apple ID, tap iCloud, manage storage and documents
Reduce system storage by updating iOS: open Settings, tap General, tap Software Update, install updates