Q: I had to turn on lost phone for my sons i phone, someone had turned off phone, the alarm sounded when they turned on phone and saw my message about police , phone was turned in to school office . How do I de activate lost phone mode, I have my password

I had to turn on lost phone for my son's I phone 5c, someone had turned phone off when they found it , but alert and message police called had them turn it in to school office when they saw message, My question  is how do I deactivate lost mode, I used a password I know but it doens't tell anywhere on Icloud.com/find how to turn off