Insert data on already existing User in mongodb

Can anyone help me solve this problem, I’ve been stuck days on it and haven’t found any solution to.
I have this user schema in my mongodb / node js:

const userSchema = new Schema({
    username:{
        type: String,
        required: true,
        unique: true,
    },
    name:{
        type: String,
        required: true,
    },
    password:{
        type: String,
        required: true,
    },
    phoneNumber:{
        type: String,
        required: true,
    },
    age:{
        type: Number,
        required: true,
    },
    helpSent: [Help.schema],
});

The helpSent is basically an array that stores every data the user has requested in the application.

Now by using my update function, which is the following:

exports.createHelp = async (req,res) => {
    try{
        const decoded = jwt.verify(req.headers.authorization.split(' ')[1], config.secret);
        const help = new Help({
            type: req.body.type,
            description: req.body.description,
            timeToRespond: req.body.timeToRespond,
            emergencyLevel: req.body.emergencyLevel,
            acceptance: req.body.acceptance,
            state: req.body.state,
            dateIssued: req.body.dateIssued
        });
        const user = await User.findOne({username: decoded.data});
        user.helpSent.push(help);

        const savedUser = await user.save();
        res.json(savedUser)

    }catch(e){
        console.log({message: e});
    }
}

I’m getting this error as if the function is storing a new data entry in the database and telling me that the username is already taken since I have it as a unique value

{
  message: Error: user validation failed: _id: Username already in use.
      at ValidationError.inspect (D:\Patrick\Flutter\Fyp\draft-v1\backend\node_modules\mongoose\lib\error\validation.js:48:26)
      at formatValue (node:internal/util/inspect:763:19)
      at formatProperty (node:internal/util/inspect:1681:11)
      at formatRaw (node:internal/util/inspect:1006:9)
      at formatValue (node:internal/util/inspect:793:10)
      at inspect (node:internal/util/inspect:340:10)
      at formatWithOptionsInternal (node:internal/util/inspect:2006:40)
      at formatWithOptions (node:internal/util/inspect:1888:10)
      at console.value (node:internal/console/constructor:323:14)
      at console.log (node:internal/console/constructor:359:61) {
    errors: { _id: [ValidatorError] },
    _message: 'user validation failed'
  }
}

Anyone help can help, and thank you!

>Solution :

Why dont you use updateOne

await User.updateOne({ username: decoded.data }, {
   $push: {
      helpSent: help
   }
})

The first argument is the query condition, in your case you search for an username with decoded.data, and the second argument is your update logic