Depending on your model, you can do it with tools or without tools:

To adjust the height of your saddle, you have to release your seatpost. The seatpost is fixed by the seatpost clamp. It is the red piece:

How to change the height?

This is the right height of your saddle

When the pedal is in the down position, your leg must be extended without effort. 

Adjusting the height of your saddle

First, it is important to know what is the right height of your saddle. Put your bike againts a wall and sit in your saddle. Put your heel on the pedal.