Start with a recoverable site
For an existing website, prepare a backup and use a staging copy before changing its theme. Record the active theme and plugins so you can reconstruct the starting configuration. A theme preview is useful for appearance checks, but does not replace testing forms, checkout or other important actions.
Choose the installable ZIP
The WordPress uploader needs the theme archive. A download that contains documentation, several ZIP files or other extras may be an outer bundle. Inspect its contents to locate the actual theme package; selecting the surrounding bundle can produce a missing stylesheet error. Do not rename an unrelated archive to style.css.
Upload and review
In WordPress administration, open Appearance → Themes → Add New, choose Upload Theme and select the installable ZIP. Install it, then activate it on the prepared test site. Follow the particular theme’s documentation for any additional components. Demo import is a separate operation and may add content or change settings.
Before changing production
Check representative pages, navigation and the site’s main action on desktop and mobile. Keep modifications outside the parent theme where practical. Once the production change is verified, retain the recovery backup according to your normal backup policy.