Simple Understandable Approach

int size = 2 * n - 1;

for (int i = 0; i < size; i++) {
    for (int j = 0; j < size; j++) {
        for (int k = 0; k < n; k++) {
            if (i == k || i == size - k - 1 || j == k || j == size - k - 1) {
                printf("%d ", n - k);
                break;
            }
        }
    }
    printf("\n");
}

just going layer by layer

if(i == k || ...

print...

break; // this is the most important part cause it only allows one layer at a time