how to add number to field in every doucment in monogdb using nodejs

my collection has phonenumber field. i want to add 91 infront of phonenumber.

{
"_id": "6137392141bbb7723",
"email": "[email protected]",
"lastname": "Cagle",
"firstname": "Brooke",
"phonenumber":9000000001
},
{
"_id": "6137392141bbe30723",
"email": "[email protected]",
"lastname": "Cagle",
"firstname": "Brooke",
"phonenumber":9000000002
}

i want to update my collection like this.

{
"_id": "6137392141bbb7723",
"email": "[email protected]",
"lastname": "Cagle",
"firstname": "Brooke",
"phonenumber":919000000001
},
{
"_id": "6137392141bbe30723",
"email": "[email protected]",
"lastname": "Cagle",
"firstname": "Brooke",
"phonenumber":919000000002
}

im trying with this code but its not updating the field.

var x = await User.find({});
        x.forEach(async function(d)
            { 
                var phone = d.phonenumber;  
                var newPhoneNumber = 910000000000+phone;
                console.log(newPhoneNumber);

                //update 
            await User.updateMany({},{$set:{phonenumber:newPhoneNumber}},
                {
                    new: true, 
                    runValidators : true
                });
            })

im getting only one value for all documents like this:

{
"_id": "6137392141bbb7723",
"email": "[email protected]",
"lastname": "Cagle",
"firstname": "Brooke",
"phonenumber":919000000001
},
{
"_id": "6137392141bbe30723",
"email": "[email protected]",
"lastname": "Cagle",
"firstname": "Brooke",
"phonenumber":919000000001
},
{
"_id": "6137392141bbe30723",
"email": "[email protected]",
"lastname": "Cagle",
"firstname": "Brooke",
"phonenumber":919000000001
}

i don’t know whether this approach is correct or not.Thank you