If you do not have access to a programmer, and for some reason do not want to run at 1/6 speed, just change the crystal to half the frequency. All timers etc run based on the 12 clock bit and a given load of e.g.TH1 and TL1 will give the same result for say 12 MHZ at 6 clocks or 12 clocks at 24 Mhz.

Erik