How can I “reset” an Arduino board?

Question

I\'ve uploaded a sketch to an Arduino Uno whose loop is something like this:

void loop(){
    Serial.println(\"Hello, World!\");
}

So, now,

Answer 1

I got a similar problem.

If I power on my Arduino, there is a delay before the uploaded program execute.

So I use that delay for uploading new program, or empty program:

void setup(){}

void loop(){}

So my problem was solved.

Unplug any connection to Arduino pins before upload.