iCosmo what do you want to reinstall ? Reinstalling usually doesn't resolve anything if you used composer the way it's supposed to be.
If you made changes to vendor, you can easily reinstall all vendor dependencies by deleting the vendor folder and running composer install again. Make sure you have composer.lock so the exact same version of everything will be reinstalled. Or delete the lock file to get the latest version of every dependency, but it's best not to do that on a production system if you're not sure what you're doing.
You can also reinstall Flarum from scratch, create the config file manually, point to your existing database and copy the fonts to assets folder (identical to beta 7 to beta 8 instructions, except you do it for beta 8 to beta 8)
The only things to copy to the new flarum are the database and assets folder. storage folder might be needed if an extension makes use of it. Some extenions might also have created their own folders in public. If you don't want to have to reinstall your extensions, copy composer.json (and composer.lock) to the new install as well.
