To upgrade, first modify the files /etc/apt/sources.list and /etc/apt/sources.list.d/raspi.list. In both files, change every occurrence of the word ‘jessie’ to ‘stretch’. (Both files will require sudo to edit.)
Then open a terminal window and execute
sudo apt-get update
sudo apt-get -y dist-upgrade
Answer ‘yes’ to any prompts. There may also be a point at which the install pauses while a page of information is shown on the screen – hold the ‘space’ key to scroll through all of this and then hit ‘q’ to continue.
Finally, if you are not using PulseAudio for anything other than Bluetooth audio, remove it from the image by entering
sudo apt-get -y purge "pulseaudio*"
After the update which took roughly 4 hours (on a Pi2 ... (why didn't I just do a fresh install?)) my wifi dongle was no longer recognised!
Anyone got any ideas?