Edit in JSFiddle

document.getElementById("button").addEventListener("click", function() {

  var input = document.getElementById('input');
  var result = document.getElementById('result');
  var url = input.value;
  var index = url.search(/[\/\/]\w*.(\/)/gm);

  var newDomain = "https://html.com";
  var newUrl = newDomain + url.substr(index);

	result.innerText = newUrl;
	
});

<input id="input" type="text" value="https://htmlforum.io/topic/63758-zamena-teksta-na-html-css-js/">
<br>
<button id="button">Update URL</button>
<br>
<div id="result"></div>