How to change the cover's icon?

I want to change the icon on the right, what should I do, how to change the source code?

No need to change the source code. You can change the icon in the configuration customize section,

Here’s an example from my customize.yaml file that changes the friendly name and icon of one of my binary sensors:

  friendly_name: Doorbell
  icon: mdi:bell

You can find other material design icons here:

I believe the request is to change the appearance of the up/stop/down control icons. As far as I know, those are not modifiable via customize.yaml.

Yeah, got my right and left mixed up.

Not changing the icon on the left, it is the button icon on the right