At first you should create a backup copy of your old CMSimple installation in a folder on your PC.
An update of CMSimple_XH is much easier, because CMSimple_XH is utf-8 encoded since version 1.2. Anyone who operates a site with CMSimple_XH, normally already operates utf-8 encoded plugins, so that the takeover of the plugin contents also should not cause problems.
Unfortunately a simply update does not exist, for that CMSimple 4.0 and CMSimple CMSimple_XH are too different.
Just install CMSimple 4.0 or higher on your local server or on the web server in a directory, and:
Now you can delete your old CMSimple installation (backup made?) and upload the new CMSimple installation to this place. Note, that on many web servers you have to assign the necessary write permissions for folders and files.
If your provider allows, you can simply forward the domain to the directory, in which you have your new CMSimple installation. Then you have not to care again for the write permissions of files and folders.
At the end, you still need to check the configuration, page title, description and keywords (for Google), language, and above all:
!!! CHANGE YOUR PASSWORD !!!