Delp are you referring to the password hash?
Flarum will use the default hasher configured for PHP
password_hash method, so if you want to use any of the PHP-provided hashers you should be able to change them directly in the PHP configuration and Flarum will honor that. Note that if you are importing PHP-generated hashes from an external system you don't need to change the hasher at all, PHP can validate any password that uses any of its available hashers even if it's not currently set as the default. If a user changes password it will then use the default hasher.
If you would like to provide a custom hasher for password validation, you can use the
Flarum\Extend\Auth::addPasswordChecker extender. You can see the documentation in the extender phpdoc https://github.com/flarum/framework/blob/v1.7.0/framework/core/src/Extend/Auth.php#L21-L40
Another option to extend the hasher, in particular if you also want to change how a hash is saved is to use a service provider to change the
hash container binding to something of your choice. The Laravel hasher interfaces are used.
If you are migrating from a different platform, I would suggest using this extension https://discuss.flarum.org/d/8631-old-passwords which will allow you to convert old password hashes to PHP/Flarum hashes on the next login, since usually when there's the need for a custom hasher it means you are dealing with outdated/insecure hashes that would make sense to convert.