If you want to change the servo every second, try to use a conditional expression in the timing structure. If you want to use more parallel structure with multiple intervals use different counters (example see: [here]).

Here is an example of the every second version:

<!-- language: c -->
 
 #include <Servo.h>
 Servo myservo;
 unsigned long previousMillis = 0; 
 const long interval = 1000; 
 int angle = 150; 
 
 void setup() { 
 myservo.attach(9);
 myservo.write(angle); 
 }
 
 void loop() {
 unsigned long currentMillis = millis();
 
 if (currentMillis - previousMillis >= interval) {
 previousMillis = currentMillis;
 
 switch (angle) {
 case 150:
 case 65:
 angle = 15;
 myservo.write(angle);
 break;
 case 15:
 angle = 35; 
 myservo.write(angle);
 break;
 case 35: 
 angle = 65; 
 myservo.write(angle);
 break;
 }
 }
 }

**EDIT:** Add variable assignment `angle` in conditional expression.

[here]:https://stackoverflow.com/questions/32037217/how-can-i-replace-delay-by-millis/32043899#32043899