I have a Small Bike which works on 6 v 4.5ah sealed lead battery by default.
as Now i am Fixing 12 v Led to modify , i choose a 12 v 2.5amp sealed acid battery which is used in bigger motorcycles .
so is it possible to convert 12v to 6v to power the bike motor ?
please let me know
It's possible I guess but not a good solution. The motor uses more power than the LED so it's easier, more efficient, and cheaper to have the motor run straight off a 6V battery and convert the 6V to 12V for the LED.
The LED is unimportant compared to the motor so the motor should get priority when choosing the battery. Match the battery to the motor, not the LED.
Don't build it or you'll just get caught up in trying to make it work. Buy it. A converter the converts from lower to higher is called a boost converter. Look for that.
I think this is a childs toy bike from the fact that it is powered by a 6 V 4.5 AH battery. If that is the case the head light will not need to be very bright. The OP does not even say what the purpose of the LED is (Head light tail light ,turn indicator.) I think this is another case of people expecting a solution to their problem without properly defining the problem.