Why would you want to do that? One of the more bizarre requests on this forum! but if you really want to do it then just upload a blank screen with a redirect to the most boring website you can think of, visitor numbers will drop pretty quickly!
If you mean decrease as in literally decrease the number i.e. improve ranking then check this thread:
Important Tips to improve Alexa Ranking of a Website
Write Quality Content.
Install Alexa Toolbar.
Content Posting.
Social Networking.
Submit Websites/Blog to Search Engine and Directories.
Work on Quality Backlinks.
Analysis your Site to Ignore Ranking Affecting Errors.
Increase Search Traffic on Your Website.
Important Tips to improve Alexa Ranking of a Website
Write Quality Content.
Install Alexa Toolbar.
Content Posting.
Social Networking.
Submit Websites/Blog to Search Engine and Directories.
Work on Quality Backlinks.
Analysis your Site to Ignore Ranking Affecting Errors.
Increase Search Traffic on Your Website.
Important Tips to improve Alexa Ranking of a Website
Write Quality Content.
Install Alexa Toolbar.
Content Posting.
Social Networking.
Submit Websites/Blog to Search Engine and Directories.
Work on Quality Backlinks.
Analysis your Site to Ignore Ranking Affecting Errors.
Increase Search Traffic on Your Website.