Filemaker doesn't have extra icons included. You can google or bing and find icons on the net. Once you paste the icon on the screen just right click then click button setup. A few of the starter solutions have icons setup as buttons.
to accomplish a) and assuming that your button is a standard FMP button:
in layout mode, select the text tool, the T symbol. double click on the button, this should select all of the text on the button. start typing your new label.
if your current buttons are actually graphics, you can replace them with a FM button creation tool and again type in your own label. NOTE: make sure you know the name of the script and any script parameters that were attached to the graphic button so you can assign them to your new button. you don't have to delete the graphic button until you have tested the new one works correctly.