Upgrading Qlicksoft CMS
Upgrading is just archive extracting into the virtual host directory where your existing Qlicksoft CMS is installed. You should always periodically backup. Failure to follow this advice will result in a lot of grief.
When upgrading Qlicksoft CMS, this will overwrite all directories and folders except for the content folder. You are strongly encouraged to perform archive backup on your entire Qlicksoft CMS installation before upgrading.
.htaccess and config.php will be overwritten. If you made any major modifications to these two files back them up.
Themes definitely need to be backed up. Everything including CSS, Templates, main.theme.php and any other files that are part of the theme package.
Everything else is core Qlicksoft CMS code. Those files should never be monkeyed with to avoid upgrade overwrites. If you made custom changes, it is up to you to remember and avoid overwrites.