How to edit data of a file File Handling C - c

Question

I'm wondering if I could use fseek() on editing the value of a variable on a file. All the tutorials I saw online were about editing a text, but how can I use fseek() to edit a value.
Here's how my program works: The user inputs a name, gender, and age. The user can input as many data as they want. Now, the user searches a name; and the gender, and age of that name will display. The user is then prompted to edit the age. After inputting a new age, the file will now be edited, with the new age written.
Here's my code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct data
{
char name[30];
char gender[10];
int age;
};
int main(void)
{
struct data client;
FILE* fp;
char ch = 0;
do
{
printf("Enter Name: ");
scanf("%s", client.name);
printf("Enter Gender: ");
scanf("%s", client.gender);
printf("Enter Age: ");
scanf("%d", &client.age);
fp = fopen("data.txt", "ab");
fwrite(&client, sizeof(client), 1, fp);
fclose(fp);
printf("continue? \n");
scanf(" %c", &ch);
} while (ch != 'n'); // continuously appends file till letter n is read;
char input[30]; // user input
printf("name?\n");
scanf("%s", input);
struct data Read;
fp = fopen("data.txt", "rb");
int newAge;
while (fread(&client, sizeof(client), 1, fp))
{
if (strcmp(client.name, input) == 0) // compare variable with user input
{
printf("%s", client.name);
printf(" %s", client.gender);
printf(" %d y/o", client.age);
printf("\n");
printf("enter new age");
scanf("%d", &newAge);
//fseek function
}
}
return 0;
}
Hope somebody can help.