First, I searched LibreOffice in Google and downloaded it on my Ubuntu. When you visit their official site, they will detect your computer system and offer you the right download.
Then, I also searched “install LibreOffice in Ubuntu 10.10” and got a very useful article http://www.muktware.com/news/21/2010/392. I opened Ubuntu Terminal window and installed LibreOffice just by following the tutorial. When the installation was done, I checked Applications- Office, and found that it has been installed on my machine successfully!
How to make it the default for double-clicking a .odt/.doc file? Well, I right-clicked an existing .doc file and opened Properties. Under the “Open with” tab, I changed the application to open files of type “Word Document” from OpenOffice to LibreOffice.
When I double-click the document, it works!