How to change the home dock icons on TouchWiz?
The way it works on my SGS2 Android 4.0 is


Click on applications,
Click menu button, select "edit"


Now the screen shows several app icons you can change, but at the bottom are also the home buttons. All home buttons except for the most right one ("start page" or "home page") can be dragged and dropped, and you can move other app icons to the row with the home buttons.

Note that this only works if at step 1 you are in grid view, otherwise change to grid view in the menu first.

If this doesn't work, there may be some additional carrier software that prevents it.