update specific attribute in mongoose

I have a production MEAN web project and I need to modify specific attribute after being created

app.post('/api/user',function(req,res){

var user = req.body;
console.log(req.body);


userModel
    .create(user)
    .then(
        function(postObj){
            var number = req.body.phone.substring(1);
     res.json(200); 
    },function(error){
        // res.sendStatus(400);
        console.log(error)
        res.json(error)
            }
    );});

Here I have to update specific attribute let's assume it's name gender and give it static value such as 'male' and I don't want to have a callback or use the return object

any help please...?