It's really simple, just type this command:
sudo dpkg-reconfigure tzdata
It will ask your living town and will setup automatically your computer time and timezone.
Notice: the timezone data are stored in these files /etc/timezone and /etc/localtime
Discussion
For manuel change ;
# Update the timezone write $AREA/$ZONE to /etc/timezone# Then copy timezone file as localtime exec(sudo rm -f /etc/localtime && sudo cp -f /usr/share/zoneinfo/$AREA/$ZONE/ etc/localtime)that is it.