Ember 2 Actions Inside Each Loop

Consider the following:

days_week: [
    {
        selected '',
        day: 'mon'
    },
    {
        selected '',
        day: 'wed'
    },
    {
        selected '',
        day: 'fri'
    }
]

Inside my template, I can loop through the days:

Which produces this: mon wed fri

What I now want to do is assign an action, so that upon click it will add a class, and upon clicking again, it will remove the class....

    <button class="" >
        
    </button>

However, the following code does not seem to work to (start by adding the class highlight)?

actions: {
    toggle: function(day_week){
        day_week.set('selected','highlight');
    }
}

and I get this error: TypeError: day_week.set is not a function ?