One of my clients is going through a rebrand and the domain needs to change ASAP. I was going to do this through my website's WordPress backend, in the general settings, and then redirect the old domain at the host level.
However when I sign into WordPress and go to General Settings, both the WordPress Address and Site Address boxes have the existing domain, but they're greyed out and not editable.
Does anyone know why this would be and how I fix it? I'm not a developer (mostly design) but there is no IT team at this small company so I hope to muddle my way through this as best as possible.

It’s likely set in wp-config.php – the AWS Bitnami stacks work that way (and likely other stacks).
Note that after changing the domain, you will also need to update all references in the DB. This can be done with either WP CLI search-replace or a plugin called Better Search Replace. (do not perform the find/replace using SQL commands)
This plugin should help you: [https://wordpress.org/plugins/go-live-update-urls/](https://wordpress.org/plugins/go-live-update-urls/)