Thanks very much for your reply, and Updating the forum post.
Just want to share with what I did.
I have copy and pasted the following part in my flarum site( all but this line “tituspijean/flarum-ext-auth-ldap”: “*” )
“require”: {
…
}
and past in the config.php in the yunohost file.
Then run
sudo -u flarum php8.0 composer.phar update --prefer-dist --no-plugins --no-dev -a --with-all-dependenciesphp
and it worked to downgrade flarum from 1.8 to 1.4.
However, as the docs in the flarum site, after update, to run the following commands:
composer update --prefer-dist --no-plugins --no-dev -a --with-all-dependencies
php flarum migrate
php flarum cache:clear
I don’t know how to run the second and third. However, after I import database.
My site is migrate to Yunohost. I have made a post, and it worked. So Yay! 
One step closer to move my sites to Yunohost server.
Can anyone tell me how to run these command in Yunohost for flarum?
flarum migratephp
flarum cache:clear