First, let's clarify that you want to modify the xtick values, not ytick.
The best approach is to have your x values be datetimes or durations. They your xticks will be time values. You can then set the values using xtick and their appearance using xtickformat.
% Create x, y data
x = (duration(0,30,0) + hours(0:23))'; % durations - no date associated with it
y=rand(24,6);
plot(x,y,'LineWidth',1.3)
xlabel('Hour')
ylabel('Concentration of ozone (in ug/cm)')
title('Diurnal variation of Ozone')
legend('Jan','Feb','Mar','Apr','May','Jun')
% Format and place ticks as desired
xtickformat("hh:mm")
xticks(hours([5.5 11.5 17.5 23.5]))
