I have the Post Grid Plugin, and I purchased and downloaded the Pro version zip file to my desktop. When I tried to install the Pro version, I received an error in WordPress:
Unpacking the package…
Installing the plugin…
Destination folder already exists. /nas/content/live/hansblake/wp-content/plugins/post-grid/
Plugin installation failed.
Return to Plugin Installer
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Just uninstall the older version and then install the pro version. your data won't lost by uninstalling the plugin. if you see any error after upload and activating please go plugin list and activate from there.
Let me know for more help.