The LM293 is a comparator, not much use here at all.
As other have pointed out, the simplest speed control is by adding resistance in series with the motor power feed. The problem is twofold, firstly, much of the excess power is dumped in the resistance and secondly, it makes the motor speed more vulnerable to the load on it. For example, adding a mechanical load to a 12V motor when it is fed directly from 6V will slow it down. Adding the same load to the same motor fed from 12V but with a resistor to drop it to 6V will slow it much more or even stop it.
The reason is that a motor will draw more current as it's load increases, if the power source can provide it, the motor input power will increase. If the resistor limits the power it can draw, the voltage goes down instead and the motor might stall.
Tell us: do you need to control the speed or the motor power? If you need to control the speed, either by analog methods or PWM, you need a feedback mechanism that controls the motor voltage from the shaft speed so it can regulate itself.
Brian.