How can I report my device stolen?

How to mark your iPhone as Lost or Stolen

1. Open the Find My app and choose the Devices tab or the Items tab.
2. Select your missing device or item.
3. Scroll down to Mark As Lost or Lost Mode and select Activate or Enable.
4. Follow the onscreen steps if you want your contact information to display on your missing device or item, or if you want to enter a custom message asking the finder of your missing device to contact you.
5. Select Activate.

https://support.apple.com/en-us/HT210515