Alex83,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
.but {
height: 50px;
width: 100px;
padding: 0;
font-size: 12px;
}
.but span {
font-weight: bold;
}
</style>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>
<script>
$(function() {
$(".but").each(function(i, el) {
$(this).data("html", this.innerHTML);
})
.mouseenter(function() {
$(this).text("подменный текст");
}).mouseleave(function() {
$(this).html($(this).data("html"))
})
});
</script>
</head>
<body>
<button class="but"> текст<br> <span>выделенный текст</span>
</button>
</body>
</html>