As per my knowledge and point of view, you should not delete any existing blog, as it could be playing a role in the SEO of your site. Updating the content regularly is always a good idea , so you may go update the content and don't delete any.

For ranking your site as a rich answer on Google, you need to work on your content and on-page optimization like your content should be such that it helps the user solving the problem or it should be comprehensive enough to resolve the user query. Work on you on- page SEO as well and keep updating your content regularly. With consistent efforts and time you will get your site as a rich answer on Google.

As per my knowledge, these days Blog Commenting is not considered as a very good technique to increase the rankings, it can be a used as medium to bring good traffic, but it is not an effective technique to increase rankings.

If you want your site to rank higher for keyword "seo site in india", then you need to implement all the on- page and off- page SEO techniques, like try inserting your keyword in every heading of the webpage , also add your keywords in the content of the page appropriately and check the speed and performance of your site on various devices.