<!DOCTYPE html>
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=utf-8" />
<title>ГГ</title>
<style>
#button {
width: 30px;
height: 30px;
}
</style>
</head>
<body>
<input type="text">
<button id="button">9</button>
<script>
button.onclick = function(){
var input = document.querySelector('input[type="text"]')
input.value += 9;
}
</script>
</body>
</html>