When using GET in forms it is necessary to pass values to inputs, like this:

<?php
$acao = $_GET['inst'];
$acao2 = $_GET['sala'];
$acao3 = $_GET['ano'];
$acao4 = $_GET['mes'];
$acao5 = $_GET['diahora'];
?>
<form action="confirmar.php">
<input name="inst" value="<?=$acao;?>">
<input name="sala" value="<?=$acao2;?>">
<input name="ano" value="<?=$acao3;?>">
<input name="mes" value="<?=$acao4;?>">
<input name="diahora" value="<?=$acao5;?>">

If you are selects you will have to do something like (this is just an example to understand and adapt):

<?php
$options = array(
    'Valor 1' => '1',
    'Valor 2' => '2',
    'Valor 3' => '3',
    'Valor 4' => '4'
);

$campo1 = empty($_GET['campo1']) ? null : $_GET['campo1'];
?>
<form action="">

    <select name="campo1">

    <?php foreach ($options as $descricao => $value): ?>

        <option <?=($campo1 === $value ? ' selected' : '');?> value="<?=$value;?>"><?=$descricao;?></option>

    <?php endforeach; ?>

    </select>

    <button type="submit">Enviar!</button>

</form>

Two simple examples to solve:

① Button with onclick event

Can be solved simply with an onclick event button

<input type="button" onclick="location.href='pagina.php?foo=1&bar=ok';" value="envia">

② Sending by POST method and rescuing $ _GET

Modify the GET method for POST.

<form action="pagina.php?foo=1&bar=ok" method="post">
<input type="submit" value="send">
</form>

Why does it work?

The reason is that the URL defined in the action attribute is always sent by the GET method, regardless of whether the action attribute is set to POST.

This mode may be the most ideal for you because it is less intrusive, meaning you will not make any changes to the codes. You just need to swap get with post in action of form