JavaScript: Replace text of an HTML element without using Id property

I got a situation where I have to replace a HTML text in production environment, which is getting generated dynamically from back-end C# code and I have to do this without rebuilding and redeploying the dlls.

I wanted to do this the simplest way, by just deploying the aspx page and replacing the content using JavaScript as show in the example below:

I need to replace a heading which is in <h2>...</h2>tag:

<h2>Test - 1</h2>
<h2>Test - 2</h2>
<h2>Test - 3</h2> 

I have to replace "Test - 3" to "Test - 5". As I know the it is the 3rd <h2> element, I used the below JavaScript code:

var string = document.getElementsByTagName("h2")[2].innerHTML;
var replacedString = string.replace("Test - 3", "Test - 5");
document.getElementsByTagName("h2")[2].innerHTML = replacedString; 

This can also be accomplished alternatively by going through all the <h2> elements one by one and replace the  matching text using an if condition.