This error happens because the theme is not at its latest version, and it appears only when major versions are released, for example 2.0.0 / 2.1.0 / 2.2.0 etc.
Note: This message is only shown for Site Administrators when logged-in, not for visitors or other subscribers.
The first thing you could to do fix this is to access Dashboard > Updates and make sure to also update Rey (theme) to the latest version. Sometimes the theme or core plugin new updates are not shown simultanously. It usually requires some sort of way of clearing the transients, for example, try clicking on “Check again” button eg:
To check the versions, please access Rey Dashboard and look for the versions status block.
If for some reason the Rey Theme update is now showing up to update, try accessing ThemeForest and download the latest Rey Theme and install it in the backend like you’d normally install a new theme. You’ll be prompted to replace the current version.