How can i edit specific part of line in a text file in C?

Question

Let's say that I have a .txt file which contains these two lines:

Item="1" Name="Sword" Damage="2.5"
Item="2" Name="Axe" Damage="3"

How can i change the damage of the Axe to 3.5 in the text file with C?

EDITED: this is how i read each line of the text file, and I'm filling a gtk list store with the text info, the dhd_LocalizaItem its a function to get what is inside the " " e.g. Item="1", i will catch the 1 with that function, but all gtk thing its working fine, as I said, i want now a C function/command to edit specific part of a line in a text file.

    void get_message() {

    GtkTreeIter dhd_iter;
    FILE * dhd_pFile;

    char dhd_G_CodProduto[256];
    char dhd_G_NomeProduto[256];    
    char dhd_contaItem[16];
    char dhd_quantidade[32];
    char dhd_valorItem[32];    
    char dhd_getbuf[1024];
    dhd_chekDel = 0;
      dhd_pFile = fopen ("Logs/logCancelar.txt", "r");

        if(dhd_pFile == NULL) {
        printf("Erro! Nao foi possivel abrir o log de cancelamento!\n");
        }
    else {
             while( (fgets(dhd_getbuf, sizeof(dhd_getbuf), dhd_pFile))!=NULL ) {

        dhd_LocalizaItem (dhd_getbuf, dhd_stringItem);
    dhd_restou = dhd_LocalizaItem( "Item=" , dhd_getbuf);
    strcpy(dhd_contaItem,dhd_restou);

        dhd_LocalizaItem (dhd_getbuf, dhd_strong);
    dhd_restou = dhd_LocalizaItem( "CodProduto=" , dhd_getbuf);
    strcpy(dhd_G_CodProduto,dhd_restou);

    dhd_LocalizaItem (dhd_getbuf, dhd_strung);
    dhd_restou = dhd_LocalizaItem( "NomeProduto=" , dhd_getbuf);
    strcpy(dhd_G_NomeProduto,dhd_restou);

    dhd_LocalizaItem (dhd_getbuf, dhd_streng);
    dhd_restou = dhd_LocalizaItem( "Quantidade=" , dhd_getbuf);
    strcpy(dhd_quantidade,dhd_restou);

    dhd_LocalizaItem (dhd_getbuf, dhd_stringTotal);
    dhd_restou = dhd_LocalizaItem( "ValorTotal=" , dhd_getbuf);
    strcpy(dhd_valorItem,dhd_restou);

    gtk_list_store_append(GTK_LIST_STORE( mainWindowObjects.liststore ), &dhd_iter);
    gtk_list_store_set(GTK_LIST_STORE( mainWindowObjects.liststore ), &dhd_iter, 
               ITEM, dhd_contaItem,
                       CODIGO, dhd_G_CodProduto ,
                       DESCRICAO, dhd_G_NomeProduto ,
                       QTD, dhd_quantidade,
                       VALOR, dhd_valorItem,
                       -1 );
          }
    }





    fclose(dhd_pFile); 
}

And sorry for my bad english everyone.

Answer 1

#include <stdio.h>
#include <string.h>

typedef struct arms {
    int id;
    char name[32];
    double damage;
} Arms;

int read_arms(Arms *a, FILE *fp){
    return fscanf(fp, " Item=\"%d\" Name=\"%[^\"]\" Damage=\"%lf\"",
        &a->id, a->name, &a->damage);
}
void write_arms(Arms *a, FILE *fp){
    fprintf(fp, "Item=\"%d\" Name=\"%s\" Damage=\"%3.1f\"\n",
        a->id, a->name, a->damage);
}

#define ARMSFILE "arms.dat"

void change_damage_by_name(const char *name, double damage){
    FILE *fin, *fout;
    Arms a;

    fin = fopen(ARMSFILE, "r");
    fout = fopen("arms.tmp", "w");
    while(EOF!=read_arms(&a, fin)){
        if(strcmp(a.name, name)==0)
            a.damage = damage;
        write_arms(&a, fout);
    }
    fclose(fin);
    fclose(fout);
    remove(ARMSFILE);
    rename("arms.tmp", ARMSFILE);
}

int main(void){
    change_damage_by_name("Axe", 3.5);

    return 0;
}