ClickAlgo when you edit config.php to point to an existing Flarum database, Flarum will automatically show you the upgrade screen next time you visit the forum. This process will convert the existing database to the latest Flarum version.
If you have a huge data set, you can run php flarum migrate from the command line to perform this action server-side, but usually the web upgrade screen should be fine.
If the test database is already at the same Flarum version as the test website, then it's a drop-in solution. Just edit config.php to point to an existing database and Flarum will immediately use that data set. You can create a copy of your existing database either before or after the upgrade and copy it to your test server. You might want to set the mail driver to "log" on the test website to make sure no email goes out to your real users from that website.
