How to change a variable with an if statement

So I'm trying to have a difficulty setting in a game I'm making in JavaScript. The hard mode has a shorter timer. I've tried shortening the time remaining variable with an if statement but it doesn't change the timer in the game.

<input type="checkbox" id="cb2" value="yes"> Hard

<script>
var timeRemain = 10;

if (document.getElementById("cb2").checked){
    timeRemain == 5;
} else {
    timeRemain == 10;
}
function countdown() {
    var timerId = setTimeout(countdown, 1000);
    if (timeRemain == 0) {
        clearTimeout(timerId);
    } else {
        timeRemain--;   
        document.getElementById("timer").innerHTML = timeRemain + ' seconds left!';
    }
}
</script>

I'm still very new to JavaScript so I'm aware I might be doing this completely wrong.