There are 2 main blocks that will be useful for variables, changing and setting the data stored inside.
The first block set allows you to set the value to anything as shown below. This is often used at the start of the code where you want it to begin at a certain value.
The second block is used to change the value by a certain amount and is often used throughout the code to change things depending on what happens. For example, losing a life.
Click on this link and try changing the numbers within the white ovals to see what happens.