Just edit the citation but the method depends on what version of EndNote you’re using. If using X5 or later, place your cursor on the citation, right-click to bring up a submenu which includes an option for displaying the Author (Year) format.
If using EndNote X4 or earlier, place your curson on the citation, right-click to display a submenu, click to select “Exclude Author” which will then display (2005). Then just manually type in “Smith et al” in front of (2005).