Update failed. The response is not a valid JSON response.

Sounds to me like there is a server side error running the update. This should be visible in the error log of your hosting – the cause would also be recognizable there. Talk to your hoster’s support if you can’t find the error log.

Alternatively, you can also enable debugging in WordPress. Then WordPress itself writes a logfile where the reason should be recognizable: https://www.ads-software.com/documentation/article/debugging-in-wordpress/

Furthermore you could download the current theme package here and upload it manually: https://www.ads-software.com/themes/twentytwentythree/ – but this won’t solve the potential problem in your project.