All the icons in Ubuntu are inside the theme which is in use.
Now to change applications icon, which you can see on the left most side of Applications.The following steps should followed:-
1.Open terminal and type gksudo nautilus.
2.Copy this location into your new window:- /usr/share/icons/
3.Choose the icon type which is in current use like Humanity,Oxygen,Humanity-Dark,etc.
4.Let us use Humanity-Dark icon as for an example,
here /24 is the image size of applications icon.
- just paste the image you made in the above location.
- start-here.svg is the name of that applications icon.
- rename your image with start-here.svg.
5.For the effect in the gnome panel again open terminal and then type:- pkill gnome-panel.
6.You can customize your applications icon accordingly.