You want to change the I am working in ABC company. to I am working in ABC company.. These are the same strings, so I don't see a reason to, but you can accomplish this by using the JavaScript innerHTML or textContent.
element.innerHTML is a property that defines the HTML inside an element. If you type element.innerHTML = "<strong>This is bold</strong>, it'll make the text "This is bold" bold text.
element.textContent, on the other hand, sets the text in an element. If you use element.textContent = "<strong>This is bold</strong>, The text "This is bold" will not be bold. The user will literally see the text "This is bold
In your case, you can use either one. I'll use .textContent. The code to change the <p> element is below.
function get_content(){
   document.getElementById("txt").textContent = "I am working in ABC company.";
}
<input type="button" onclick="get_content()" value="Get Content"/>
<p id='txt'>
<span class="A">I am</span>
<span class="B">working in </span>
<span class="C">ABC company.</span>
</p>
This, unfortunately, will not change it because it'll change it to the same exact text. You can chance that by changing the string "I am working in ABC company." to something else.