Obtain the schematic of your Flasher LED circuit. Examine the components on it and determine which resistor(the "timing resistor") controls the flash rate. Check on the schematic to see if the timing resistor or "flash rate" resistor are marked as such. Contact the designer or manufacturer of the circuit to find out the location of the timing resistor if you can not locate it yourself.
Multiply the value of the timing resistor by two to obtain the value of the timing resistor that will double the length of the LED flasher.
Disconnect the power to the LED Flasher circuit. Measure the voltage levels on each of the components and discharge any charged components (such as high voltage capacitors) if you are working with a circuit that can store dangerous levels of electrical current.
Remove the timing resistor from the circuit and replace it with a resistor that has twice its resistance value.
Turn on the LED flasher and observe that the time between flashes has been lengthened. Increase the value of the timing resistor if the time between flashes has not increased enough. Reduce the value of the timing resistor if the time between flashes has decreased.