Around couple of weeks ago my website's home page was appearing in Google top 10 SERPs for keyword ABC but an internal page of my website is appearing in Google SERPs for keyword ABC and I noticed that my ranking on Google for keyword ABC has drooped to 3rd page from 1st page. Just want know is it due to change in ranking URL?
Is there any way to tell search engines that for keyword ABC my main page is home page not internal page.
This sounds like the change that Google made to their algorithm several weeks ago. For many sites I know of, it was an improvement - because their internal page is a better landing page for that keyword than the home page, which covers a wider sweep of topics.
If this algo change is what affects your rankings - then it sounds like in your case the internal page is not so good. Maybe you'd want to "de-optimize" it a bit. However, that leaves open the question of what happened to your domain root. Is it still indexed?
Well, I've never worked the process in this direction - I've always had the home page ranking when an internal page was better. I'd probably begin by analyzing why Google like the internal page so much and move some of those factors to the home page.