In order to use the Volume enum, the Television class would need to look something like the following (only Volume related functionality shown)

class Television {

   public static final Volume DEFAULT_VOLUME = Volume.MEDUIM;
   
   private Volume volume;       // the volume

   Television() { this(DEFAULT_BRAND, DEFAULT_VOLUME); }   
   
   Television(Volume volumeIn) { this(DEFAULT_BRAND, volumeIn); }
   
   Television(String brandIn, Volume volumeIn) {
      this.setBrand(brandIn);
      this.setVolume(volumeIn);
   }
   
   public void setVolume(Volume volumeIn) { volume = volumeIn; }
   
   public Volume getVolume() { return volume;  }
   
   // returns a string representation of this class instance
   public String toString()
   {
      return "Television: brand=" + this.getBrand() + ", volume=" + this.getVolume();
   }
   
}

The toString() method that outputs volume would now show something like:
Television: brand=RCA, volume=MAX

In the setVolume() method, there is nothing you need to do to validate incoming data - the method parameter is of type Volume, so the compiler will catch any attempt to set it to something other than a Volume value

To add another volume value, you'd just need to change the enum, e.g.
public enum Volume {
  OFF, VERY_SOFT, SOFT, MEDUIM, LOUD, VERY_LOUD, 
  WAY_TOO_LOUD,
  MAX
}
