#include <stdio.h>

#define N  100

char a[N][N];  unsigned int n;

void try(unsigned int i, unsigned int j)
{
    if (i < n && j < n && a[i][j] == '+')
        a[i][j] = '-', try(i-1,j), try(i+1,j), try(i,j-1), try(i,j+1);
}

int main(void)
{
    unsigned int i, j;

    if (scanf("%u", &n) != 1 && n > N) return 1;
    for (i = 0; i < n; i++)
        for (j = 0; j < n; j++)
            if (scanf(" %c", &a[i][j]) != 1) return 1;
    
    for (i = 0; i < n; i++)
        try(0, i), try(n-1, i), try(i, 0), try(i, n-1);
    
    printf("%u\n", n);
    for (i = 0; i < n; i++) {
        for (j = 0; j < n; j++)
            putchar(a[i][j]);
        putchar('\n');
    }
    return 0;
}

> void try(unsigned int i, unsigned int j)
> {
>     if (i < n && j < n && a[i][j] == '+')
>         a[i][j] = '-', try(i-1,j), try(i+1,j), try(i,j-1), try(i,j+1);
> }

ちょっと見たところ、i, j が負になりませんか？

>   if (scanf("%u", &n) != 1 && n > N) return 1;

訂正です。
    if (scanf("%u", &n) != 1 || n > N) return 1;