What's the correct way to update to a new version of Dataform? I don't want to lose the Dataform activities I've already created.
What's your current version?
I currently have the September release and want to ugrade to the latest October release. I'm running Moodle 2.2.
Very good. Upgrade should be done just like upgrading moodle. If you installed the Dataform by git clone or by git remote tracking you can simply pull the master to upgrade. if you installed it by extracting the folder from the zip into your Moodle mod directory, you can remove the current folder and extract the one from the Oct. build into the mod directory (don't just paste the new over the old one because this will not remove obsolete files; you need to remove the old one first). Then go to notifications to complete the upgrade. All current data should persist. Highly recommended to backup everything so as to be able to rollback if the upgrade fails for some reason and to try the upgrade first on a testing environment. I have received reports on successful upgrades so it should work.
Please note that removing the current module folder from the mod directory does not mean deleting the plugin via the admin plugins managment page. Do not delete the plugin via the admin ui. If you delete the plugin from there it will delete all your current module activities and their content.