<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button class="btn1">Поменять текст</button>
    <button class="btn2">Текст</button>
</body>
<script>
    let btn1 = document.querySelector(".btn1")
    let btn2 = document.querySelector(".btn2")
    btn1.onclick = function(){
        btn2.innerHTML = "Новый текст"
    }
</script>
</html>

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External JavaScript

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