While repeat structure to recalculate in C

The problem is that you are not restarting the value of the res variable when it will recalculate:

#include <iostream>
using namespace std;

int main() {
    int vetor[10], i;
    char op;
    do {
        float res = 0; //agora toda vez que for repetir o pedido vai zerar a variável
        for (i = 0; i <= 9; i++) {
            printf("\nInforme um numero: ");
            scanf("%i", &vetor[i]);
            if (vetor[i] % 2 == 0) {
                res += vetor[i];
            }
        }
        printf("\nA soma dos pares e: %.2f", res);
        printf("\nDeseja calcular novamente? ");
        scanf("%s", &op);
        fflush(stdin);
    } while (op == 's' || op == 'S');
    return(0);
}

See running on ideone .

Some small problems in the code besides this but that does not stop the operation.

The problem is that the res variable does not have its value reset at the next iteration of the do..while loop. This can be corrected in two ways

Moving the declaration of the res variable into the do...while loop:

do {

    float res = 0;

    for (i=0; i<=9; i++) {

Note that in this way the correct one is to remove the previous declaration of the variable res , since it will not be used due to differences in scopes.

2- Restarting its value at every iteration of do...while through an assignment only:

do {

    res = 0;

    for (i=0; i<=9; i++) {

Note

As res receives integer values, it does not have to be declared as float , and it is sufficient to declare it as int or as long , if you believe its values will cause integer overflow . p>

I hope I have helped.

You can also use while as follows:

#include <stdio.h>

#define MAX 10

int main(void)
{
    int vetInt[MAX], soma_par, i, op;           

    while(1)
    {
        soma_par = 0;
        printf("\nInforme 10 numeros inteiros.\n");

        /*Leitura dos numeros inteiros.*/
        for (i = 0; i < MAX; i++)
        {
            printf("\nInforme o %i° numero: ", i + 1);
            scanf("%i", &vetInt[i]);
        }

        /*Soma dos numeros pares.*/
        for (i = 0; i < MAX; i++)
            if (vetInt[i] % 2 == 0)
                soma_par += vetInt[i];

        printf("\nSoma de todos numeros pares é %i\n\n", soma_par);

        printf("\nDigite 0 para finalizar o programa ou 1 para continuar: ");
        scanf("%d", &op);

        if (op == 0)
            break;
        else
            system("clear");
    }

    return 0;
}

In this case to close the loop and require the user to type 0 , so the loop will be terminated by break; .