If you want to update the displayName of this user:
var db = firebase.database(); db.ref("-Users/-KUanJA9egwmPsJCxXpv/displayName").set("New trainer");
Alternatively, you can also get the same with:
db.ref("-Users/-KUanJA9egwmPsJCxXpv").update({ displayName: "New trainer" });
But it's likely you don't know the ID of the user, in which case you need to look that up first:
var query db.ref("-Users").orderByChild("uid").equalTo("jRXMsNZHR2exqifnR2rXcceEMxF2"); query.once("child_added", function(snapshot) { snapshot.ref.update({ displayName: "New trainer" }) });
One final remark on your data structure though: you seem to be storing user profiles, but you're storing them under a push ID. For this type of structure we usually recommend that you store each user under their UID:
-Users jRXMsNZHR2exqifnR2rXcceEMxF2 displayName:"Test Trainer" email:"test@gmail.com"
With such a structure you remove any chance that you're storing the same user twice. Plus, you can now update the user's display name without needing a query:
var currentUser = firebase.auth().currentUser; db.ref("-Users/"+currentUser.uid).update({ displayName: "New trainer" });

If you want to update the displayName of this user:
var db = firebase.database(); db.ref("-Users/-KUanJA9egwmPsJCxXpv/displayName").set("New trainer");
Alternatively, you can also get the same with:
db.ref("-Users/-KUanJA9egwmPsJCxXpv").update({ displayName: "New trainer" });

But it's likely you don't know the ID of the user, in which case you need to look that up first:
var query = db.ref("-Users").orderByChild("uid").equalTo("jRXMsNZHR2exqifnR2rXcceEMxF2"); query.once("child_added", function(snapshot) { snapshot.ref.update({ displayName: "New trainer" }) });
One final remark on your data structure though: you seem to be storing user profiles, but you're storing them under a push ID. For this type of structure we usually recommend that you store each user under their Unique ID:
-Users jRXMsNZHR2exqifnR2rXcceEMxF2 displayName:"Test Trainer" email:"test@gmail.com"
With such a structure you remove any chance that you're storing the same user twice. Plus, you can now update the user's display name without needing a query:
var currentUser = firebase.auth().currentUser; db.ref("-Users/"+currentUser.uid).update({ displayName: "New trainer" });