The FieldTrip code is accessible using git at github.com. To quickly get access to the code, you would do the following from the command line or the equivalent in a graphical git interface, such as the GitHub desktop.
git clone https://github.com/fieldtrip/fieldtrip
To update automatically, you could schedule a job (e.g. using cron) to run “git pull” every night.
Please see our github tutorial for details.
You can also download the latest (“master”) version as a zip file.