you probably need a function to recalcuate the showed price, triggered when the slider is moved and also when the button is clicked, which uses bioth slider position and the toggle fo the button to calculate the price and print it to the page, here you have too much stuff nto connected, you need to have together both logics
var slider = document.getElementById("myRange");
var output = document.getElementById("valeur");
var output2 = document.getElementById("valeur2");
var output3 = document.getElementById("billing");
output.innerHTML = slider.value;
output2.innerHTML = slider.value;
function yearBilling (){
output.innerHTML = slider.value * 9;
}
slider.oninput = function (){
output.innerHTML = this.value;
output2.innerHTML = this.value * 6.25;
}