stm32 dynamic timer settings change

Im trying to make an device to receive the signal from external sensor (pressure sensor). I'm using HAL libraries on STM32F3Discovery and USB - USART adapter to communicate with computer through serial port. Im sending and receiving data to/from the device, but I need some way to change timer prescaler and autoreload value (to change a sampling rate) with a button on my gui application. How can I do that? I was trying do something like that:

void HAL_UART_RxCpltCallback(UART_HandleTypeDef* huart) {
if (Recived == 0) {

    HAL_GPIO_TogglePin(GPIOE, GPIO_PIN_11);
    TIM7->PSC = 119;
    TIM7->ARR = 5999;

}

But it obviously didnt work :D Help.

EDIT:

I think im doing something wrong with syntax, cause firstly I initialize tim7 like this:

static void MX_TIM7_Init(void)
{

 TIM_MasterConfigTypeDef sMasterConfig;

  htim7.Instance = TIM7;
  htim7.Init.Prescaler = 9999;
  htim7.Init.CounterMode = TIM_COUNTERMODE_UP;
  htim7.Init.Period = 719;
  if (HAL_TIM_Base_Init(&htim7) != HAL_OK)
  {
    Error_Handler();
  }

  sMasterConfig.MasterOutputTrigger = TIM_TRGO_RESET;
  sMasterConfig.MasterSlaveMode = TIM_MASTERSLAVEMODE_DISABLE;
  if (HAL_TIMEx_MasterConfigSynchronization(&htim7, &sMasterConfig) != 
HAL_OK)
  {
    Error_Handler();
  }
   }    

And then Im trying to reconfig tim7 in my main function like this

void HAL_UART_RxCpltCallback(UART_HandleTypeDef* huart) {
if (Recived == 0) {

    HAL_GPIO_TogglePin(GPIOE, GPIO_PIN_11);
    TIM7->PSC = 119;
    TIM7->ARR = 5999;
    TIM7->EGR = TIM_EGR_UG;

}
    MX_TIM7_Init();
if(Recived == 1){

    HAL_GPIO_TogglePin(GPIOE, GPIO_PIN_12);
    TIM7->PSC = 9999;
    TIM7->ARR = 719;
    TIM7->EGR = TIM_EGR_UG;

}
}

Im sure that im sending data and receiving it on uC, because of leds.