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 thy case
%F %R (or, equivalently,
%Y-%m-%d %H:%M)), or do it direct 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 (for clarity, I'm referring to the nesting of single (') quotes inside double (") ones) are necessary due to dconf's idiosyncracies. Note also that to include seconds, one can replace
(I do not recall which release it was that 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 be accessing the dconf database via
gsettings set than
dconf write. Same database, but a tool more intended for end-users (though the corresponding GUI-ish tool remains