Firstly, switch it off and unplug it from the socket. Then fill the reservoir humidifier with water and white vinegar. Keep it there for sterilizing for 30-60 minutes. Once it is done, make the tank empty and fill it with fresh water. Keep cleaning it with water till the smell does not go away. Once it is gone, use a small soft cloth to dry it completely. This method is only applicable to portable humidifiers. For the fixed ones, you can get help from the professionals.