Yes the green "handles" ...
have a look in the manual chapter "Selecting, resizing, moving and rotating".
Now for your problem :
Press the ">" button (in red on the picture)
In vertice mode, in the front view, left-click on the green square (in red again ...) and drag your mouse to the right till it has no more effect :
Now in the left view, select the 2 vertices at the left of the plane and drag them on the right (the same as above) :
The same for the vertices at the right of the plane :
