First, back up your site. Either use the database backup tool in the Manager (Tools -> Backup) and download all the files with FTP, or install
BkupMODx (which is my preferred way of backing up; this module is the first thing I install after any fresh MODx installation).
Unless you modified any of the default snippets, modules or plugins that was installed with your original, you can just upload the new files and the "install" folder, and run the installer.
Upgrading will not have any effect on your existing content in the database; it will update any of the items (snippet code, plugins etc) you check on the installer options page. Do NOT check the "demo site" option at the top of that page; this will overwrite your entire site content (as it says on the section for that option).
It’s a good idea to clear your site cache and log out of the manager first.
Oh, and did I mention back up your site first?
☆ A M B ☆
