Increase Peltier module voltage
You have a misconception what a peltier module does. It doesn't "create coldness" or sets a certain temperature but rather transports heat from one side to the other.

The amount of heat transported in one second is given in the datasheet of the module in J/s and it is a function of the electrical current running through it. The temperatures are a result of that heat transport. This current depends on the voltage, but it cannot be increased above the spec current because the module would overheat then. (This current itself creates about 10 times more heat than the heat transported from the cold side.)

If you want the cold side to be colder, the easiest way is removing heat from the hot side more efficiently, for example by using a bigger fan.