How to convert kitchen timer code into a countdown
As a start, you need to initalise the secondes and minutes variables to 30 and 1 respectively.
replace signed short minutes, secondes; with:
signed short minutes = 1;
signed short secondes = 30;

Next thing I notice is that the code you have provided counts up and you have requested that it count down.
Change secondes++; and minutes++; with:
secondes--;
minutes--;

Lastly you need to change the logic for when the secondes and minutes variables reach zero.
If the secondes variable reaches zero but the minutes variable is greater than zero, then you decrement the minutes (using the line of code provided above) and top-up the secondes with 59.
secondes = 59;

You will need to check the value of the secondes and minutes variables using a couple of if...else statements.
if (secondes == 0 &amp;&amp; minutes == 0) {
    // insert your timer finished code here
}
else if ( secondes == 0 &amp;&amp; minutes &gt; 0) {
    minutes--;
    secondes = 59;
}
else {
    // Error - your code should not reach here.
    // Display an error message on the LCD
}

This if..else statement will need to go at the top of your loop instead of the end.