I'm no expert in Ubuntu. On modern distributions there should be a directory /user/share/X11/xorg.conf.d/ with templates for various settings which once have been made in the xorg.conf file.
Chose one of the templates (on my system it would be 50-synaptics.conf) and copy it to /etc/X11/xorg.conf.d/ then you can edit and modify it to your needs, it is selfexplaining.
If you need more information about the possible settings for your touchpad, you may execute
in a terminal.
After changing the file in /etc/X11/xorg.conf.d/ you'll have to restart the X-server!