How to check if that data already exist in the database during update (Mongoose And Express)

Typically you could use mongoose validation but since you need an async result (db query for existing names) and validators don't support promises (from what I can tell), you will need to create your own function and pass a callback. Here is an example:

var mongoose = require('mongoose'),
    Schema = mongoose.Schema,
    ObjectId = Schema.ObjectId;

mongoose.connect('mongodb://localhost/testDB');

var UserSchema = new Schema({
    name: {type:String}
});

var UserModel = mongoose.model('UserModel',UserSchema);

function updateUser(user,cb){
    UserModel.find({name : user.name}, function (err, docs) {
        if (docs.length){
            cb('Name exists already',null);
        }else{
            user.save(function(err){
                cb(err,user);
            });
        }
    });
}

UserModel.findById(req.param('sid'),function(err,existingUser){
   if (!err && existingUser){
       existingUser.name = 'Kevin';
       updateUser(existingUser,function(err2,user){
           if (err2 || !user){
               console.log('error updated user: ',err2);
           }else{
               console.log('user updated: ',user);
           }

       });
   } 
});

UPDATE: A better way

The pre hook seems to be a more natural place to stop the save:

UserSchema.pre('save', function (next) {
    var self = this;
    UserModel.find({name : self.name}, function (err, docs) {
        if (!docs.length){
            next();
        }else{                
            console.log('user exists: ',self.name);
            next(new Error("User exists!"));
        }
    });
}) ;

UPDATE 2: Async custom validators

It looks like mongoose supports async custom validators now so that would probably be the natural solution:

    var userSchema = new Schema({
      name: {
        type: String,
        validate: {
          validator: function(v, cb) {
            User.find({name: v}, function(err,docs){
               cb(docs.length == 0);
            });
          },
          message: 'User already exists!'
        }
      }
    });

Another way to continue with the example @nfreeze used is this validation method:

UserModel.schema.path('name').validate(function (value, res) {
    UserModel.findOne({name: value}, 'id', function(err, user) {
        if (err) return res(err);
        if (user) return res(false);
        res(true);
    });
}, 'already exists');

For anybody falling on this old solution. There is a better way from the mongoose docs.

var s = new Schema({ name: { type: String, unique: true }});
s.path('name').index({ unique: true });