Open the Configuration Editor, it should be in Applications>System Tools, if it's not there do
yum install gconf-editor -y
from a terminal to install it. Once you get it going navigate to apps>panel>menubar and double click on the line that reads "custom icon" then enter the full path to the icon you want used, you may need to experiment with sizes so that it looks right.
OR just overwrite the start icon in whatever icon theme you are using, they are usually in /usr/share/icons.