First step is actually installing version 7 using the install guide from our documentation page. Once you have version 7 installed then you just need to download the threads_import.zip file from the Members area. In here you'll have a threads_importer.php script that you place into your importers directory. Bring this up in your browser and it will ask you some information on your 6.5.5 database to do the import. Once you fill that in it will begin importing all of your data into 7.0.