abb1=Plot[u,{u,0,1.01},DisplayFunction->Identity];
abb2=Plot[3 u (1-u),{u,0,1.01},DisplayFunction->Identity];
abb=Show[{abb1,abb2},AxesLabel->{"u_n","u_(n+1)"},
ImageSize->{400,300},DisplayFunction->$DisplayFunction];