MG-811 CO2 sensor

Your best bet is to take many measurements, and to then take the average of those. For instance:

var sum = 0;
var n = 100;
for (var i=0;i<n;i++) sum+=analogRead(INPUT_PIN);
var result = sum/n;

There are other ways you can do this as well, for instance you can median filter:

var arr = new Uint8Array(100);
for (var i=0;i<arr.length;i++) arr[i]=analogRead(INPUT_PIN);
arr.sort();
result  = arr[arr.length/2];