The turbo mode is not present in all Air Conditioners. If the AC contains the turbo mode then you will be able to see the Turbo mode button there. If When you press this button, the program run for a limited time period giving maximum cooling speed and fan speed is also placed maximum. You cannot change the temperature during this time i.e. it goes in automatic mode.
When to use TURBO mode ?
You can press this at any time. When the time period of Turbo Mode ends, it will go back to the mode where it was working before. Every AC contains the time period, where the compressor starts and to reduced this time period you can switch ON the Turbo mode so as no to wait for the compressor to start. But if there fluctuations in light, do not use TURBO mode at this time because the gases in compressor need some time for proper working.
Do not use TURBO mode repeatedly, there will be load on the compressor and Air Conditioner will stop working.