There is no Select All icon or a button for that purpose on any of Word's
default toolbars. As Suzanne points out there are several convenient ways to
Select All, plus Triple+Click in the left margin.
Perhaps you actually want to add a custom button for that purpose? If so go
to Tools>Customize - Commands. Select Edit in the list of categories, locate
Select All in the list of commands and drag it to whichever toolbar you wish.
It will be labeled automatically by name, but if you want an Icon you can
right-click the button, point to Change Button Image & pick the Icon you
want. Then, if you want the Icon only, right-click the button again & choose
either Text Only (in Menus) or Default Style.
HTH |:>)