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To change a specific character in a StringBuilder object, use the method `setCharAt(int index, char ch)` provided by the StringBuilder class.

The `setCharAt(int index, char ch)` method has two parameters in the method signature: - `index`: the position of the character we want to change; it is zero-based, which means the first character position is represented by 0, the second character position is represented by 1, and so on - `ch`: the new character we want to replace the existing character with To use the `setCharAt()` method, call it on the StringBuilder object while passing the index and the new character as parameters. For example, if we have a StringBuilder object named `myString` and we want to change the character at the third position (index 2) to 'X', the usage would be: `myString.setCharAt(2, 'X');` After using the `setCharAt()` method, the specified character in the StringBuilder object will be changed to the given character.