Change POST data

Hello, everyone, i send data by post for example phone = 1234567

var_dump($_POST['phone']); // 1234567
var_dump($this->request->getPost('phone')); // 1234567

then chage it value to 123456

var_dump($_POST['phone']); // 123456
var_dump($this->request->getPost('phone')); // 1234567


How can i change value in $this->request->getPost('phone') and in formvalidation as well?