There's no easy way to do it in the GUI. One can either use
dconf-editor (which is GUI-ish), and edit two keys in
custom-time-format to the desired
strftime directives, in your case
%F %R (or, equivalently,
%Y-%m-%d %H:%M)), or do it directly from the terminal with the following two commands (which, just to be clear, change the same keys):
dconf write /com/canonical/indicator/datetime/time-format "'custom'"
dconf write /com/canonical/indicator/datetime/custom-time-format "'%F %R'"
Note that the doubled quotes are necessary due to dconf's idiosyncracies. (For clarity, I'm referring to the nesting of single quotes (
') inside double quotes (
").) Note also that to include seconds, one can replace
I do not recall which release started including the dconf tools by default, so if they are unavailable, simply install the
dconf-tools package and proceed.
Update 2016-05-17: While the above should still work, as a matter of principle it's probably better to access the dconf database via
gsettings set than
dconf write -- same database, but a tool more intended for end-users. The corresponding GUI-ish tool remains