Kinetic energy is E

 $\frac{1}{2}m{v}^2=E\Rightarrow v=\sqrt{\frac{2E}{m}}$

Velocity at highest point $=v\cos 45°$

 $\therefore v\cos 45°=\sqrt{\frac{2E}{m}}.\frac{1}{\sqrt{2}}=\sqrt{\frac{E}{m}}$  
[ $\because$  at the highest point there is only horizontal velocity]

 $\therefore$ K.E. at highest point  $=\frac{1}{2}\times m\frac{E}{m}=\frac{E}{2}$