No reason why not. So long as the physical size of the battery matches, and the voltage it delivers is the same - you should be able to replace it with the higher mAh battery.
If they're the same voltage and chemistry - yes
Sure, if they have the same voltage.
can use 1400mah battery to replace 760 mah
No
Yes and no. The higher capacity battery will last longer, but you need to consider the charger. It depends on the design of the circuit.
mAh stands for milli ampere hour and it is a measurement of how many milli amps of current a battery is capable of producing in one hour. Yes you can substitute a battery with a higher mAh current rating provided you use the same battery voltage.
Not a good idea. It will work but not as long as the 1000 mAh battery.
Yes as long as both the 2450 mAh battery and 600 mAh battery have the same voltage. The larger one will just last longer and take longer to charge up (I am assuming these are the solar powered type lights).
In general, no. The charger is designed for a specific battery profile, and it might not be able to properly charge and/or monitor the amount of charge if you put in a different battery.
No, a 1000 mAh battery should not replace a 400 mAh battery in solar lights unless the system is designed to handle the higher capacity. Using a battery with a significantly higher capacity than what the system is designed for can potentially damage the circuitry and reduce the efficiency of the lights. It's best to stick with the recommended battery capacity for optimal performance.
With a charger specifically designed to charge this battery.
Yes, it mAh or Ah is how long the battery lasts, it does not affect performance.