what is the importance of the resistors that connect to push buttons ? can i replace them by wires?
They are what is known as Pull-Up resistors.  The circuit will not work if you replace them with wires.  Not only that but you will short circuit the power supply when you press a button.

Why don't you want to use resistors?

The value is not important - you could use the same value as you are using for the LED if you don't have the 10k resistors shown in the drawing.

Having said that an alternative is to leave them out and instead activate the internal pull-ups within the processor on the Arduino:

pinMode(pin, INPUT);           // set pin to input
digitalWrite(pin, HIGH);       // turn on pullup resistors