WordPress will soon allow updating an installed theme or plugin by uploading a ZIP file. The ZIP upload and update feature is slated for WordPress 5.5.
Until now, the themes or plugins manually installed via ZIP file could be updated via manual server uploads via FTP or hosting file manager only, unless it was a part of the WordPress repository or the developer had coded the update functionality.
WordPress allowed to you install a new plugin or theme from a ZIP file, but it didn’t allow upgrading them using a ZIP file. If you tried to upgrade from a zip file, you get an error message saying “destination already exists” and the upgrade failed. WordPress will now allow you to upgrade from a ZIP file.
Updating a theme or plugin works in the same manner as uploading a new one. When you upload an existing theme or plugin, the new core function will show up a message that the theme or plugin is already installed. WordPress will show a review screen comparing the existing installed version with the uploaded one.
Plugins such as Easy Theme and Plugin Upgrades by Chris Jean and Update Theme and Plugins from Zip File by Jeff Sherk has another 20,000. The community owes the developers of these plugins at least a small bit of thanks for taking on a job that should have long ago been a part of the core experience.