You cold make them brighter by replacing the resistor with one a smaller the value of the one there, but at the risk of blowing the LEDS. This link will enable you to decode the colour code on the resistor http://en.wikipedia.org/wiki/Resistor_color_code
If you replace the LEDS then you need to calculate the current required to flow through each LED, which you will have to get from the data sheet for the LED.
You will then need to caclulate the value of the resistor to give you the required current
The general formula is R = V/I
where R is the resistor value (ohms)
V is the voltage (Volts)
and I is the current (amps)
So if the current required is, say 5mA (0.005 Amps) and you have a 12 volts supply then the resitors value would be 12/0.005 = 2400 ohms. The nearest resistor values would be either 2200 or 2700 ohms.
2200 ohms will give you a brighter result as marginally more current will go through the LED
You will have to make sure the resistor is rated correctly for the power dissipation. The power dissipated is given by the formula W=V*I where W= Watts
In the example above the power is 0.005 * 12 = 0.06 watts, so an off the shelf 1/4 resistor would be fine, however if the current required is 1 amp then you would need a 12Watt resistor, and thats a fairly meaty item.