If you don't have the "find my iPhone" app download it from the app store.
Tap on settings
Tap on icloud
Enable find my iPhone
Press on the home button, find the "find my iPhone" app and tap on it
Enter your apple Id and password
Here you will have a list of all your devices that have find my iphone enabled. Tap on the blue arrow next to the device you want to remote lock or wipe.
Here you will be able to remote lock or remote WIPE BE EXTREMELY CAREFUL AS REMOTE WIPE WILL DELETE EVERYTHING ON THE SELECTED DEVICE