User.save is not defined [ discord.js and mongoose ]

Question

Here is my bot main code which is related to the error

global.mongoose = require('mongoose')
const config = require('./settings.json')

global.Guild = require("./data/guild.js");
global.User = require('./data/user.js');


    let random = Math.floor(Math.random() * 5)
    User.money += random;
    User.xp++
    User.messages++

    if(User.xp >= config.upXP){
      let embed = new Discord.MessageEmbed()
      .setColor(config.color)
      .setDescription(`[:tada:] Congratulations to **${message.author.username}** on your new level!`)
      message.channel.send({ embeds: [embed] });
      User.xp -= config.upXP;
      User.level+=1
    };

    User.save();
    

    if (!message.content.startsWith(Guild.prefix)) return;
    const args = message.content.slice(Guild.prefix.length).trim().split(/ +/g);
    const cmdName = args.shift().toLowerCase();
    const command = client.commands.get(cmdName) || client.commands.find(cmd => cmd.aliases && cmd.aliases.includes(cmdName));
    if(!command) return;
    if(!require('./settings.json').owner.includes(message.author.id) && command.public === false) return;
    command.execute(bot, message, args, config);
})

This comes as a error

User.save(); ^

TypeError: User.save is not a function

I tried doing the markModified() function but it gave me null value . None of google solutions worked. I am working on replit and nodejs 16 .