Works way to update binary (harder then you think)

myWay.c

#include <stdio.h>
#include <stddef.h>

typedef struct {
    char name[128];
    int max_hp;
    int current_hp;
} Charactor;

#define SAVE_FILENAME "gamedata.bin"

int main()
{
    int new_max_hp = 9999;
    FILE *fp = fopen(SAVE_FILENAME, "r+b"); //This is the mode you want.
    int www;

    if (!fp) {
            perror(NULL);
            return 1;
    }

    size_t max_hp_offset = offsetof(Charactor, max_hp),
           max_hp_size = sizeof(int),
           max_hp_remaning = sizeof(Charactor) - max_hp_offset - max_hp_size;

    printf("%d %d %d\n", max_hp_offset, max_hp_size, max_hp_remaning);

    while (fread(&www, sizeof(int), 1, fp)) { //triggering EOF. Not the best way, but meh.
        fseek(fp, max_hp_offset - sizeof(int), SEEK_CUR);
        fwrite(&new_max_hp, max_hp_size, 1, fp);
        fseek(fp, max_hp_remaning, SEEK_CUR);
    }

    fclose(fp);
    return 0;
}

originalWay.c

#include <stdio.h>
#include <stddef.h>

typedef struct {
    char name[128];
    int max_hp;
    int current_hp;
} Charactor;

#define SAVE_FILENAME "gamedata.bin"

int main()
{
    FILE *fp = fopen(SAVE_FILENAME, "r+b"); //This is the mode you want
    Charactor c;

    if (!fp) {
            perror(NULL);
            return 1;
    }

    while (fread(&c, sizeof(Charactor), 1, fp)) {
        c.max_hp = 9999;
        fseek(fp, -sizeof(Charactor), SEEK_CUR);
        fwrite(&c, sizeof(Charactor), 1, fp);
        fflush(fp);
    }

    fclose(fp);
    return 0;
}