Hi, I am starting to work on a site who has 139 keyword. Is it OK to work on all the keywords altogether or do I have to chose less keywords initially. My second question is if a site which has lots of SEO mistakes like less keyword density and improper title but doing good on Search engine, If we correct the errors or change the content will it effect the ranking of the site.
I have never heard of anybody starting out with 139 different keywords that have nothing in commen with each other. That is way too ambitious. Your going to be running in place a very long time trying to rank for that many root keywords. Start out with a few.
I have heard of many people going in and doing on page seo to a site that is already ranking and they lost rank. If you are doing well then be very careful and make changes slowly. The changes you talk about are very minor anyhow. Don't over do it.
Your not giving us very much information. Your idea of bad SEO might actually be good SEO. Be very careful what you read on the Internet much of that information is outdated. You can do one thing and hurt your ranking while another person does the same thing and they improve. It is not simple. Google seems to get spooked when you make a bunch of changes to a website.
I just want to do some basic changes like using keywords in Title and including site maps. I always believe that for getting top in Google we just need to put some keywords, a quality content and good backlinks. You are right that most of the information on the internet is outdated. Also there are many confusion among other Professional SEOs regarding different things. Like some say keyword density is not important and some say it should not be exceed more than 5%. I decided to listen all but use my common sense.