To "hack" a game means, in short, to cheat in the game in order to achieve certain results. Minecraft can be modified in several ways, given below.
Steps
Method 1 of 3: How to use codes in the game
Step 1. Create a new world
Step 2. Make sure the option to use codes is enabled
Step 3. In the game, press English
the letter t to open a command window.
Step 4. Enter the command to manipulate the game
For example, enter "/ time set 0" to change the time of day to sunrise
Method 2 of 3: Making yourself invulnerable
Step 1. Open an inventory editor such as INVedit
Step 2. Put the armor on your player
Step 3. Set the damage to a very small negative value (eg -40000)
Step 4. Save the world
Step 5. Open the saved world
Step 6. You will be invulnerable while wearing armor
Note: You will be vulnerable to falls, fire and water
Method 3 of 3: Other options
Step 1. Find tutorials and download the dedicated Cheat Engine
Step 2. Install mods that allow you to make changes to the game, for example INVedit or MCedit.