What do you mean by 'tweak'? Replace? Increase? Decrease? In the appropriate circumstances any of those can help. Jeesuz - if your content is as badly written as your question, delete it all and start again.
When doing any tweaking I like to keep it around 2% density. It can be real beneficial to use related keywords too! Title, first paragraph and last paragraph should have the keywords and the rest throughout.
A lot of people get confused on this. So many people scream at others, "Change your keywords." Not as simple as that. You can't just change your keywords. That is, you can't take your site and turn it into something that it is not. Your content will dictate what true "keywords" google thinks your site targets. To change google's thinking, you need to completely change your content. Sometimes this is easy, sometimes it's hard. You can tweak the keywords in your niche, but you can't change your niche without a complete re-do. If your site is on home loans, you can't change the keywords to student loans. Many people want to go for higher paying keywords, but they have locked themselves into their niche. The ways to "change" (tweak) keywords, is to make sure your targeted keywords/key phrase is in your title, and it is in heading tags, <h1>, <h2>, etc. Also bolding keywords helps as well. (Many people mistake <h1>, as a header. It is not. It describes the font size, but points out importance of the following section as well.) But, it really boils down to content. The above can help if you tweak the content as well. My advice always is to just increase traffic for the keywords you have. Why try and fool people? Increasing traffic for those looking for your current keywords is really the best way.