Slowing down blinking rate on an LED with built in IC
Googling "Kingbright L-796BSRC-B datasheet" finds the datasheet for this device.  On page 2 it shows that \$f\$ is 1.5 to 3Hz for a voltage of 3.5v to 14v, and the graph on page 3 shows that the rate decreases with voltage. So increase the applied voltage, and the rate will slow.

But you won't get much change by moving from 12V to 14V, so it's time to pick a different LED.