how to use setInterval in vue component

this is not pointing to the Vue. Try

todo: function(){           
    this.intervalid1 = setInterval(function(){
        this.changes = ((Math.random() * 100).toFixed(2))+'%';
        console.log (this.changes);
    }.bind(this), 3000);
}

or

todo: function(){  
    const self = this;          
    this.intervalid1 = setInterval(function(){
        self.changes = ((Math.random() * 100).toFixed(2))+'%';
        console.log (this.changes);
    }, 3000);
}

or

todo: function(){  
    this.intervalid1 = setInterval(() => {
        this.changes = ((Math.random() * 100).toFixed(2))+'%';
        console.log (this.changes);
    }, 3000);
}

See How to access the correct this inside a callback?