2.9 is out for a day or two and already the first pages of this forum section are filled with "Help, 2.9 upgrade failed etc etc". Probably less than 10% has a problem but then panic mode is on, because you did it on your live site and now you are losing visitors or even income.
So, test it on a test site first. Gives you all the time of the world to try it out.
If you are allowed more than one database, just create a new wptest database, install wp (same version as live) and make it identical to your existing live site.
If you are only allowed one database at your host you can create an extra set of tables with another prefix and install wp in a new directory.
Don't forget the plugins. Content can be moved via XML export/import.
And always read this before hitting the Automatic Upgrade Button:
Quote: Automatic Upgrades do fail sometimes, though, so remember to backup your database first, and deactivate your plugins before starting the upgrade.
And I might add, check first whether your plugins are fit for the Wp version that you are planning to install. Otherwise: wait.