Just open the site in FW5 and select ‘Save archive…’ from the File menu. It’ll create a new folder with a copy of the FW document, a folder containing all used media and a empty site folder. Move this archive to your upgraded configuration and open in it FW6.
Depending on whatever actions you’ve used you MIGHT run into some little things, but in general you should be fine. Just make sure you also install the third party actions on the new configuration, but check if they have been updated for FW6. If so … install the FW6 ready actions.