Can I change Siri's accent?

Set the voice (back) to what you want: Change Siri voice or language - Apple Support

Re-download the voices as needed, too.

As for updates, or network glitches, or gremlins with our passcodes changing some setting somewhere, yeah, that occasionally happens. Unfortunately. If you can figure out how to reproduce these changes, let Apple know.

Finnerly77 wrote:

Why is it that Siri now has an American accent? I specifically chose something not American… Updates shouldn’t change things like this.

There are several Siri voices to choose from. Apple may reset your selections randomly. Apparently, that's something that Apple has started to do recently. All you can do is keep a backup and change it back after Apple breaks it.