Notes

//start updatePhone function
void updatePhone(void)
{
FILE*stockptr;

char inputCode[50], pCode[50];
int newstock, stock, option;

//Open the file with read and write mode
stockptr = fopen("stock.txt", "r+");

//Ask for the model
printf("nEnter the phone model whose stock quantity you want to change and current stock: ");
scanf("%s%d", inputCode, &newstock);

//Repeat the process of scanning each string from file until End of File (EOF) : You can use 'fscanf'
while(!feof(stockptr))
{
fscanf(stockptr, "%s%d", pCode, &stock);

//compare the string with the modelCode : You can use 'strcmp'

//if the model is matched with the string
if(strcmp (pCode,inputCode)==0)
{
//Get the current file pointer : use 'fseek'
fseek(stockptr, 0, SEEK_SET);

//Jump the pointer until it reached before the stock quantity value

//Write and replace the value with new value
stock=newstock;

// exit the loop

}//end if

}//end the loop
fclose(stockptr);

printf("n");
menu(option);
}