Hey Fauzanul,
You can use the '\mathbf' command for specific parts of the string that need to be bold. Here is an example of how to use it:
figure;
plot(1:10);
ylabel('$\mathbf{|\frac{\ddot{X}(i\omega)_{2}}{F(i\omega)_{4}}|}$ $\mathbf{(\frac{mm/s^2}{N})}$', 'Interpreter', 'latex');
Hope this helps!
