Take a look at the crawling rules section of the Console when logged in. This allows sections or pages to be re-ranked or completely omitted from the index based on Rules. Using Rules, a Boost factor between -50% and +50% can be applied to change where a page ranks relative to other results in queries. It's very quick and easy to have your entire site re-ranked in minutes.
Remember though, Sajari is a self-learning engine that optimizes each search based on how your users interact with results. This means your results, and subsequent click through rates, will improve automatically with time. If result sets aren't what you would expect, you may want to look at your content. Does it match the language of your users? Could you improve page titles, descriptions, or body content? Have you overlooked canonical or noindex tags that may exclude key pages from results? We generally find it more useful to do a thorough content audit before setting Boost rules or similar.