How do I reverse this following circuit, i.e. when the laser is off then it should stop buzzing?
Just swap your CDS component and the \$6.8k\Omega\$ resistor.

When the laser lights the CDS its resistance becomes very low, so the 2N7000 gate is tied to ground, so the mosfet is off and the buzzer is off too. When the CDS is not lit the \$6.8k\Omega\$ resistor ties the mosfet gate to \$V_{cc}\$ turning it on and turning on the buzzer (and the led).

If you swap the CDS and the \$6.8k\Omega\$ resistor the behaviour of your circuit will be inverted as you request.