To negate a condition, right click it and select 'invert condition.'
Altogether it should look something like
+Mouse is over button
-Set frame to 2 (assuming that's highlighted)
+Mouse is NOT over button
-Set frame to 1 (assuming that's not highlighted)
The only problem with that is if you highlight one button then move the mouse directly to another instance of the same button then it might not go back to frame 1.