From the project description, it seems like 2A is a pin on a 74SN00N NAND gate IC from Texas Instruments.

The 2A is the 'A' input for the second NAND gate in the IC (the IC has four 2-input NAND gates).

Therefore, it's not something that can be 'replaced' in the normal sense. If it is damaged or not working somehow, you'd need to replace the entire IC.

But if you're looking to use a different input for the NAND gate, you could use any other unused 'A' or 'B' input on one of the other NAND gates in the IC. Just remember that in a 2-input NAND gate, both inputs must be high (1 logic level) for the output to be low (0 logic level).