Look up the chain rule for partial derivatives, and the equations that give you [itex]x, y, z[/itex] in terms of [itex]r, \theta, \phi[/itex] for spherical coordinates. Use these to re-write the derivatives [itex]\partial^2 \psi / \partial x^2[/itex] etc. into the derivatives [itex]\partial^2 \psi / \partial r^2[/itex] etc. There's a lot of algebra. The final result (which you should be able to see in your textbook) contains both first- and second-order derivatives.