Arduino question (sketch update)

Question

Arduino

I thought of making one device ... Everyone is talking about Arduino, it's useful to read, it seems to fit in all respects.

But I can’t find an answer to one question anywhere ... so I made a device, sold it to a certain number of people, and then I found a bug in the sketch. Well, it is necessary, respectively, to give people a new firmware. Of course, they should not fall into the hands of the text of the sketch. It is also obvious that the process should go without installing the Arduino IDE. Optimally - an exe-file, by running which, when the arduino is connected, the user will update the sketch in it by pressing one button. Actually the question is: how?

Well, in passing, I’m a complete zero in electrical engineering, but knowledgeable people will still read the topic, maybe someone will tell you ...
Here is a diagram:

1. What is the triangle EB in the lower left corner, what does it symbolize? More precisely, the main question - does the whole circuit manage the ground wire? Or is there still tension because of this mythical triangle? =)
2. I need to wedge into the circuit at points 3 and 5, closing this circuit with an arduino. For this purpose, I found this: www.nuelectronics.com/estore/index.php?main_page=product_info&cPath=7&products_id=32 Did I understand correctly that this is exactly what I need? Will this relay work if the wire is not power, but ground?
3. Let's say the relay is on, the circuit closes and opens, everything works. I really want to use the Arduino to catch situations when switches are pressed in an existing circuit (Left-Up or Right-Up). If there was voltage going there, then it seems like I could take a wire from point 5 and lead it to the input of the arduino. And how to catch the appearance of the earth? Mind explodes =)

Answer 1

Update: use avrdude like (huh?) the ArduinoIDE does. True, it will be a flash driver + firmware + batch file. But can we pack the whole thing in one place?
2. The relay closes the contacts, regardless of whether it is ground, or phase, or who knows what else. Use a relay+diode for significantly less money.
3. Not enough conditions. As a spherical horse in a vacuum, I propose to put a leg lift through a 10k resistor to Vcc. When the button is pressed, it will be 0, when it is released - 1. In the program, you invert the logic.

Answer 2

About pull-up resistors it became clearer, that's good. Yes, the arduino has internal pull-ups, they can be used if they are pulled up to the supply voltage (I honestly don’t remember).
It is quite possible to find out, measure the voltage between points 3 and 5. If there is 5V, everything becomes quite obvious - just throw a wire from point 5 to the controller foot. If 12 or something else, you will need to put a transistor that will broadcast high voltage signals to low + 5V.
Take the circuit with a transistor from this one, you need a “speedometer, tachometer” block.
Pulling up to different voltages, of course, is not good. fraught with fireworks)
Measure what states can be at point 5, let's start from this.