The fix from another thread. QUick and easy…
Depending on whether the update refers to WordPress, a plugin or a theme, it may help to flush out the transient data..
3 options exist in the options table..
_site_transient_update_core
_site_transient_update_themes
_site_transient_update_plugins
Flush out whichever is misreporting an update..
If a plugin.. _site_transient_update_plugins
If a theme.. _site_transient_update_themes
If WordPress.. _site_transient_update_core
By flush, i mean drop/remove that option(only that option) from the options table.