The rule that avoids the loss is a single one: whatever you edit inside the theme or plugin folder will be wiped out at the next update. It’s not a bug and it comes with no warning: updating replaces the entire folder.
If you already made edits and haven’t updated yet, copy the changed files to a safe place before proceeding.
Theme: use a child theme
The child theme is a separate folder that inherits everything from the parent theme and holds only your changes. It’s never updated: the parent is. Your changes survive.
Many themes in the catalog already ship with a -child.zip inside the package. If it’s there, install both and activate the child: you’re done.
If it’s not there, creating one is quick. A folder in wp-content/themes/, say meutema-child, with a style.css file starting like this:
/*Theme Name: My Theme ChildTemplate: meutema*/
The Template value has to be exactly the parent theme’s folder name: that’s the number one mistake here, and the symptom is WordPress saying the main theme is missing. Along with that, a functions.php that loads the parent’s CSS, and the child is functional. Activate it under Appearance > Themes.
After that: new CSS goes in the child’s style.css; new functions go in the child’s functions.php; to change a template, copy the file from the parent to the child keeping the same path and edit the copy.
CSS: a child theme isn’t always necessary
For a purely visual tweak, Appearance > Customize > Additional CSS stores the code in the database, and it survives any update. It’s the simplest solution when the customization is just cosmetic.
Plugin: don’t edit the plugin
A plugin has no equivalent to the child theme. What to do instead:
- Use the hooks the plugin exposes, from your child theme’s
functions.php. That’s the mechanism the authors provide precisely for this. - If the plugin allows overriding templates from the theme (WooCommerce is the best-known example), use that route, which is built to survive an update.
- As a last resort, a small custom plugin of your own with your change. It’s untouched when the other one updates.
Before updating, even with everything set up right
Backup and, if the site matters, a test environment. A child theme protects what you wrote; it doesn’t protect against the new version from the author changing something your site relied on. If that happens, you can roll it back.
