What is sending data to the Processing application?
data = port.readStringUntil('.');
So, what is in data now?
data = data.substring(0, data.length() - 1);
Make a substring of all the characters in data, and store in data. How useful is that?
index_Power = data.indexOf(",");
Suppose data doesn't contain a ,. What will be the value in index_Power?
Power = data.substring(0, index_Power );
Does is make sense to always call this function? What is the output if index_Power is -1?
Etat_Arduino(Power,112,103); //print : "ERROR !" instead of "OK !"
What does this comment mean? If you want the function to always print "ERROR!", remove the code that does otherwise.
I would also like to change the variable that the program of the arduino send me, for example, it sent me "Power = 1" and I want to "Power = 0" so I sent it 0, and I will add another variables, how can I do for the arduino to receive. and processing sent it?
Is the Arduino the master, and the Processing application the slave? If so, it doesn't make sense for you to be modifying what the Arduino sent, and telling it it was wrong.
The Processing sketch that you posted is not expecting an input string that contains "Power = 1" or "Power = 1" or any variation that contains the string "Power =". There is nothing in the Processing application that changes the value in Power, except for that code that sets it based on what it receives from the serial port.
You haven't posted the Arduino code at all.
So, it isn't clear how to help you.