Arduino thermometer and humidity display with 7 segment LED display
You need to maintain a state which tells your program whether you are displaying the temperature or the humidity.  Every time the timer expires you need to change from one state to the other, and then display the relevant value depending on what the state is.

Something like:

bool displaying = false; // false = temperature, true = humidity

void loop() {
    if ((millis()-timer) &gt;= INTERVAL) {
        timer = millis();
        display = !display; // Switch between display values

        if (display) {
            float h = dht.readHumidity();
            sevseg.NewNum(h * 100, 2);
        } else {
            float t = dht.readTemperature();
            sevseg.NewNum(t * 100, 2);
        }
    }
    sevseg.PrintOutput();   
}