Just open a Terminal window and copy/paste the following command:
gsettings set org.gnome.shell.extensions.dash-to-dock click-action 'minimize'
Hit enter for the change to take effect instantly.
If you don’t want to lose the ‘window’ picker (option 5 in the list above) run this command instead, which will introduce minimise on click but show the window picker if more than one window of a given app is open:
gsettings set org.gnome.shell.extensions.dash-to-dock click-action 'minimize-or-previews'
To undo the change (i.e. revert back to the default settings for the Ubuntu Dock) copy/paste this command:
gsettings reset org.gnome.shell.extensions.dash-to-dock click-action
