You've had Roundcube up and running for a while, and now there is a shiny new version out and you need to upgrade. Where to start?
Backup, Backup, Backup!
I can't stress this enough: Back everything up!
First, make a copy of your Roundcube directory. This will help in case the new version does not work for you, you can always go back.
Next, perform a database dump (schema and data) and save the file in a safe place. If the new version of Roundcube requires any changes to the database schema, you may need to restore this backup if you have to revert to the earlier version.
On the command line
Get the latest version
Download it from http://www.roundcube.net/download and copy the Tarball to a directory on your server.
Unpack the tarball and read UPGRADING and INSTALL files and check system requirements of the new version.
cd <where-the-tarball-is-saved> tar xf roundcubemail-*.tar.gz
Update your existing Roundcube installation
The easiest way to do this is to use the installto.sh shell script bundled with Roundcube.
cd <the-unpacked-roundcube-directory> bin/installto.sh <your-existing-roundcube-directory>
Follow the instructions you'll see in the shell. The script first copies all updated files to the target directory and then runs the update script that will update/migrate your local config files and update the database schema if necessary.
Via FTP + Installer
Get the latest version
Download the files for the newest version first. The easiest way to get it is to visit http://www.roundcube.net and click on "Downloads" or just go to http://www.roundcube.net/download. Wait for it to download, and then unzip (using WinZip or "tar xvfz roundcubemail-*.tar.gz") in the current directory.
Now you should read UPGRADING and INSTALL files and check system requirements of the new version.
Upload the new files
Use your favorite FTP/SFTP/SCP program to upload the updated files, which are:
Upload plugins/* and skins/* from the release package but don't replace the entire skins and plugins folders!
Also copy the default config file and the mimetypes mapping:
Run the installer
Edit your Roundcube config (config/config.inc.php, or config/main.inc.php for versions < 1.0) and set 'enable_installer' to true. Then open http://<url-to-roundcube>/installer/ in your web browser and click "3. Test config"
Follow the instructions on the screen to update your local config and the database schema.
When you're done and all the lights are green in the installer, edit your Roundcube config file again and set 'enable_installer' to false if it's still present. To seal your installation, you should even remove the installer directory from the webserver.
Updating from < 0.9 to 1.0
With version 1.0 of Roundcube, the structure of the config files changed. The previously used main.inc.php and db.inc.php files are now replaced with one single config.inc.php file which only contains the config options that differ from the defaults. This shall make future updates even simpler. Both, the command line update script as well as the installer will take care of the migration. Just note that from now on, the (one and only) config file for Roundcube is config.inc.php.
That should be all that is needed to upgrade. If you have problems, you can always restore the backups and try again, or ask for help on the forums. If you need help, be sure to take note of any errors that appear on your screen or in the log files.
There's also UPGRADING file in Roundcube package to which you should refer to. Wiki can be outdated.