How do I modify the value of a single element in an array?

So i have an array that has a size that is based on the users input (order0) and the cells up to (order0-1) are all randomly generated and the last cell must be a value that when added to the sum of the (order0-1) cells equals a certain number... here is the method i am having trouble with:

public static int[] sumsarray(int s, int o){

int order0=o*2;

int sumall = 0;

int lastsum;

int sumArray[];

int lasti = order0;

//creates the one dimensional array for the sums

sumArray = new int[(order0)];

Random generator = new Random();

for (int i=0; i<(order0-1); i++){

sumArray[i] = generator.nextInt(s/order0);

}

for (int i=0; i<(order0-1); i++){

System.out.println(sumArray[i]);

}

// sums all the arrays up until the last value

for (int i=0; i<(order0-1); i++){

sumall += sumArray[i];

}

System.out.println("the sum of the first sums is:" + sumall);

lastsum = s - sumall;

System.out.println(" the last sum is:" + lastsum);

sumArray[order0]=lastsum;

return sumArray;

my question is how do i get the last sumArray value to be equal to the lastsum. thanks in advance