Step 1 : Backup your existing files of wordpress site .
Step 2 : Backup your wordpress database.
Step 3 : Deactivate your plugins. A plugin might not be compatible with the new version, so it’s nice to check for new versions of them and deactivate any that may cause problems. You can reactivate plugins one-by-one after the upgrade.
Step 4 : Download WordPress 2.6 (~1.2MB)
Step 5 : Delete your old
wp-admin directories (I would suggest you to rename it till the time WordPress 2.6 is installed successfully).
Step 6 : Copy the new WordPress files (after unzipping) to your server, overwriting old files in the root. You may use FTP or shell commands to do so. Note that this means *all* the files, including all the files in the root directory as well. If you use the default or classic theme and have customized it, then you can skip that theme.
NOTE : Take care to preserve the content of the wp-config.php file in the root directory. This file contains current settings for your existing installation, e.g. database sign-in information.
Step 7 : Login to the Admin section and Point your browser to “http://yourdomain/wp-admin/upgrade.php“. Follow the instructions on screen to continue and upgrade within seconds.
Congratulation, Your WordPress site is upgraded to version 2.6
Step 8 : Delete the renamed folders, if had not deleted yet (Step 5)
In case any thing goes wrong you still have the backup taken in Step 1 and Step 2 to restore to your old version.