You can't directly upgrade the two. My understanding of that is that the upgrade reuses as many things as possible, and reusing 32 bit components would defeat the purpose.
The way I'd do it is to create a backup of your home directory, install Ubuntu 64-bit, then restore the home directory from the backup. Because most, if not all, of your settings are in the home directory, everything should be preserved. You'll then need to reinstall all of your applications, but that should be fairly straightforward.
For the backup, turn on hidden files (Ctrl-H), select all of your files (Ctrl-A), and copy them to an external drive. You should unplug the drive before the installation for safety reasons. Once you're done with the install, copy the files back. You'll need to turn on hidden files again if you want your settings to be preserved, since most of them are in hidden directories.