Generally you don't. The purpose of a PDF file is to provide a universally distributable file. If you want to edit, you change the originally source document then regenerate the PDF.

If this was not your document, you need to get the original source file from the author.

Adobe Acrobat: Adobe - Adobe Acrobat: Create PDF file, edit PDF file, convert PDF to word, convert PDF to doc

You could either:
a) with Adobe Acrobat from http://www.adobe.com/products/acrobat/
Or
b) get the open source software called PDFedit for free. Follow the installation instructions for Your operating system on source-forge.net - it might be little trickier to get to work then Adobe Acrobat :)
Good Luck.
PS: Here's a bigger list of PDF editing and reading Software on wikipedia.org