It is about 2 month since I am working for my site, now I want to change some of my keywords. Is this harmful for ranking?

I might be wrong but I wouldn't of thought so.

yea sure you can change the keywords, anyway you are changing it because, they do not get you good spots in SERPs rite?? if yes, change them for better and see that you submit the new revised pages with new titles and descriptions to google for indexing via webmasters. Be patient to see the latest result, you might have to wait for weeks.

If you mean the keywords meta tag, it hasn't been used by search engines for years. Google has never used it. Keywords in your title tag, h1 tag and page content are the primary on-page source of keyword ranking. The description tag is important for click through - a meaningful description in a search result will encourage people to click it. That's a massive over-simplification but it's a good place to start. There are many sources of information that will get you up to speed with the hundred or so things you need to do every day to get a high ranking website. Start with the help pages in Google Webmaster Tools.

You can change your keywords whenever you want but be aware of distributing that new keywords well in your content.

keyword is effect your website. so try to use good keyword to find in search engine. Google adwords will find good keyword for your site.