if you can't seem to get anywhere on the iPhone itself you can also remove it from the iCloud.com website.
1. Turn the iPhone Off. If the device is on and Find My iPhone has an active location, it will not allow you to delete it.
2. From a computer, go to www.icloud.com and sign in using the iCloud ID that is currently tied to the iPhone in question.
3. Now click on Find My iPhone.
4. Click on All Devices at the top of the Find My iPhone screen.
5. Find the device in question in the list and click on it.
6. On the next screen, click on Remove from Account. You may get a popup over the map instead, if that's the case, refer to below.
7. One more popup will come up asking you to confirm. Click on Remove.
Note: If you got a popup with device information on the map instead of the device information screen, verify the device is Off and Find My iPhone isn't finding a location for it. If that's the case, follow these steps:
1. Click on All Devices at the top of the Find My iPhone screen one more time.
2. Find the device in the list again and this time you should see a gray x appear to the right of it. Click it.
3. A popup will appear asking you if you're sure you want to remove the device. Click Remove.
Now turn the iPhone back on.