Erase Data on my stolen phone without password I’ve lost an iPhone without a passcode and had put it in lost mode at the moment. However, I would like to erase the data too as I do not want the thieves to see the contents on my phone. But when I press ”erase data” on Find iPhone，I was prompted for my iCloud password and phone passcode even though I had no passcode set. I suspect that the thief has probably set a passcode himself. How do I erase the data on my phone this way? Thanks.
It's sad you learned a lesson the hard way.

Because the phone had no passcode, the thief (or anyone else who happens upon it) can see contacts, photos etc. When you get a new phone, it is highly advised that you add a passcode. The time and inconvenience of unlocking when you use the phone is worth the security. 

As for the stolen device, It is recommended that you should at least change your Apple ID password.  Instructions can be found here at Apple Support.  I is unlikely that the thief will guess your old password, but if the current password is weak or can be easily guessed (or worse, you have it plain to see in the Notes app) he could do even more damage to your personal data.  If you have any other Apple device that uses your Apple ID this could be a serious problem.  He could even purchase apps at your expense, change the password himself and lock you out.

If you are still concerned and you have your original proof of purchase, a call to Apple Support may be an option.