What's happening here is that you're changing the potential energy of this object by lifting it 500cm.
Therefore the energy required to lift it would be the additional potential energy it gains. Look at the equation for potential energy:
PE = m*g*h
and convert and plug in your values:
PE = (2.5kg) * (9.8 m/s²) * (0.5m)
PE = 12.25J
