Turning off Find My iPhone on ruined device

I just had to replace my son's iPhone using my equipment protection policy from my provider. Before sending back the old one (ruined by water damage), the insurer says to turn off Find My iPhone. The phone won't power on at all, so I think turning it off may be impossible. Can I do it through iCloud? And if so, how? I logged in to iCloud and it offered no help in this. Thanks!