setAttribute()

js/script.js

function funMake() {
document.getElementById('b1').setAttribute('value', 'OK');
}

function funMake() {
document.getElementById('b1').setAttribute('value', 'OK');
}

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<script src="js/script.js"></script>
</head>
<body>
<form id="f1" action="#" method="post">
<input id="txt1" type="text" />
<br />
<br />
<input id="b1" type="submit" name="perform" />
</form>
<br />
<script>
funMake();
</script>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<script src="js/script.js"></script>
</head>
<body>
<form id="f1" action="#" method="post">
<input id="txt1" type="text" />
<br />
<br />
<input id="b1" type="submit" name="perform" />
</form>
<br />
<script>
funMake();
</script>
</body>
</html>