I have purchased a Honda Shine SP 2016. It is a bit tall, my feet don't touch down completely. My friend suggested to change the shock absorbers. Can someone advise me about this?
Feet touching the ground completely is not required to ride safely, provided you can balance the bike steadily when stopped.
I would advise against lowering your bike if at all possible. If it is unsafe for you at that ride height, I would suggest you purchased the wrong bike.
If you choose to change the ride height, do both front forks and rear shock absorbers in order to keep the bike level. Depending how much you want to lower it, you may need to purchase different components. This could get very expensive, if the parts are even available. Simply lowering one end could cause control issues.
The disadvantage to lowering the bike is it changes the design from how it was intended to function. These changes could degrade the ride quality, handling and, depending how you ride, contact with the road when cornering.