Here is the step by step instruction of Themes & Plugins update

Step 1: Go to your My Account page, you’ll get all of your Downloadable products on Download Section. You can download an update from there.

Step 2: Go to Appearance > Themes > Add New (For Theme) or Plugins > Upload Plugin (For Plugins).

Step 3: Upload your Theme or Plugin there. Then Click Install Now. Your Theme or Plugin will be updated.

Troubleshoot

N:B: If you get this type of error, then simply delete that existing theme/plugin from your directory. Then reinstall it.
Or, Install this plugin if you want to upgrade themes and plugins using a zip file without having to remove them first.