How do I upgrade Moodle? Do I just overwrite the files?
Do not overwrite files, it may cause strange errors. You should read the Upgrading documentation before proceeding.
I obtain the message "Upgrade already running in this session, please wait!"
Most likely you refreshed the page before the completion message. If you are absolutely sure that there are no upgrade processes active (php and/or mysql), you can click on "!!!" and restart the upgrade.
- Note: If you click on "'!!!" or try to restart the upgrade from another browser, there is a chance that your data in the database could be corrupted. If this happens, you will need to restore the database from sql dump and then restart the upgrade and wait - the process can take several hours on large sites.
I've just upgraded to Moodle 2 and notice lots of missing files
Check whether the files were also missing in the Moodle 1.9 site. Often problems, which on first sight seem to be caused by the upgrade, actually existed in the 1.9 site.
Can I upgrade more than one version at a time?
See the section 'Upgrading more than one version' in Upgrading.