WordPress has a built-in feature to install themes and plugins by supplying a zip file. Unfortunately, you cannot upgrade a theme or plugin manually if it already exists. Instead, WordPress will say “destination already exists” when trying to upgrade using a zip file and will fail to upgrade the theme or plugin.
Manual update process can be done well easy now, with the use of Easy Theme and Plugin Upgrades plugin. First install and activate this plugin, then follow the below steps upon your need of theme or plugin update.