Hi :)
The easiest way is to just drag&drop the file onto the LibreOffice icon.
Yes, it is best set during the install process but it is also possible many other ways. I know of 2 in Xp, not sure how that translates to Win8.
1. Rerun the installer. It asks if you would rather "Repair" or "Modify" LibreOffice instead. Either of those options soon gives you the screen where you set whether LibreOffice is the default app for read "Word", "Excel" and "Powerpoint" (err and "Visio"). Just tick all 4 boxes and then "Next". I often miss that screen so i would have to rerun the installer again (or hit the back button).
2. As Tanstaafl says but with a twist. There is often a tick-box at the bottom of the selections window. Tick it if it says something like "Always use this program to open this file-type". It is a bit painful though because there are quite a few different types called "doc" and "docX" that would all normally open in Word and some of them are extremely rare. Sadly until you have opened a lot of MS files in LibreOffice this method wont work even without the twist because Windows tries to hide the fact that LibreOffice is installed.
3. Open a folder, any folder, at the top of the window choose the "Tools" menu, choose the "Folder Options" and in the pop-up click on the "File Types". Wait half an hour for it to list all the 'different' types and then click on the grey header at the top that says "File Types" (top marks for originality (NOT)) to sort the list into human-readable and then scroll down to all the appropriate types. Again it is 1 type at a time. Click on the type, click on the button "Change" and hope to see LibreOffice in there.
The 2nd way also works on Ubuntu except that from the right-click menu you would go down to Properties at the bottom instead of "Open with" and then find the right tab and select the right radio-button.
As Tanstaafl says, it is best to set at install (or reinstall-modify/repair)
Regards from
Tom 