Hello
We have JMS 1.1.21 installed on Jommla 1.5.10.
We try to update via ftp with a new joomla version 1.15.18
Following a problem we restore the old configuration

I have not seen in the faq how to perform this update
Thank you for helping me

When JMS can not install the patches, it automatically restore the backup it performed when you have installed the patches.
As the configuration.php is part of the files patched, it restore you previous configuration file.

If it did that, this probably mean that you had an error to install a patch and by security, it restore the backup to allow you login in your master.

The are plenty of potential reason and one of them is a file that could be protected (read-only).
This may happen if you have installed the joomla patches with a root login user.

Ok I restored the old version of Joomla.

I find it difficult to update when you have a joomla version JMS.
Sites must be stopped and uninstalled JMS