Mongodb – How can I accept user input if he didn't change his username?

I’ve been struggling with what logic should I put on this one. I want the user to update his information, but I’m encountering a problem.

What if the user wants to change his/her email, but keeps his username? In this case, since his username is already existing in the backend. vice-versa

Like example, I have this data.

"username": "test1",
"email": "[email protected]"

The user only wants to update his username and save his default email.

"username":"test1updated",
"email":"[email protected]"

And since I’m using a validation that,

const userExist = await User.findOne({username})
const emailExist = await User.findOne({email})


if(userExist){
     res.status(400).json("User Already Existing...")
}else if(emailExist){
    res.status(400).json("Email Already Exists...")
}else{
      try {
            const newUser = new User({
                username: req.body.username,
                email: req.body.email,
                studentid: req.body.studentid,
                password: CryptoJS.AES.encrypt(
                  req.body.password,
                  'secret_key',
                ).toString(),
              });
            const savedUser = await newUser.save()
            res.status(200).json(savedUser)
    
        } catch (error) {
            return  res.status(400).json("Error Login")
        }

 }

This means that it will not save since its existing and my mongoose only accepts unique input, How can I allow the user to update his chosen information, like update username only without changing the email?

Some might suggest, just remove the validation for email, but I also want the same function to email.

Or should I make the username unchangeable? since it’s my login information?