You can use the built-in update system, in the Help menus select "Check for Updates" and it should prompt you to update to Firefox 4.0.1.

https://support.mozilla.com/kb/Updating+Firefox

Click Help | Check For Updates. The update procedure is automatic.

Here are a few tips to make FF4 look like the previous version just in case you don't like the new layout.

• Right click a blank part of the tab bar and click "Tabs on top" to remove the checkmark. Tabs will subsequently reappear below the search bar.
  
  
• Repeat the same steps, but click the top one called "Menu bar". This will reinstate the text links at the top.
  
  
• To rearrange the layout, repeat the same steps again and choose "Customize". A panel will open and while that remains onscreen, you can move elements like toolbars, buttons etc around and place them anywhere you wish. You can also drag anything you don't like into that panel and add anything you want by dragging it out of there.
  
  
• To replace the status bar at the foot of the page, install this add-on: https://addons.mozilla.org/en-US/firefox/addon/status-4-evar/