You assign a value to an array slot using the index on the left side of the equals sign:
int[] scores = new int[3];
scores[0] = 90;
scores[1] = 85;
scores[2] = 78;
You can also overwrite an existing value the same way:
scores[1] = 95; // was 85, now 95
Unlike strings, arrays are mutable. When you change scores[1], you modify the original array in place. No new array is created. Any other variable pointing to the same array will see the updated value. I'll cover that shared-reference behavior in more detail when you reach the OOP section.