Can’t change button color using JavaScript. Error: Uncaught TypeError: Cannot set properties of undefined (setting ‘backgroundColor’)

I am a beginner in JS so can’t resolve. Please help.
I can’t change the button background color.

Here’s the code ->

<!DOCTYPE html>
    <html lang="en">
    
    <head>
        <meta charset="UTF-8">`enter code here`
        <meta http-equiv="X-UA-Compatible" content="IE=edge">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <title>JS Game</title>
    
        <link rel="stylesheet" href="style.css">
    
    </head>
    
    <body>
        <button class="btn1" onclick="change()"></button>
    </body>
    
    <script>
        function change() {
            document.getElementsByClassName('btn1').style.
            backgroundColor="lightblue";
        }
    </script>
    
    </html>