You must first remove the trial version. This is done in the WordPress Dashboard:
Plugins > Installed Plugins

Under “SP Client Document Manager Premium Trial”, click “Deactivate”

Now that it is no longer active, you may proced to “Delete” it. And click “Yes, Delete these files” to confirm.

DO NOT DELETE THE FREE VERSION: “SP Project & Document Manager”

Now you may go to “Plugins > Add New” and click “Upload Plugin”.

“Choose File” and select the zip file (you should have recieved an email from “Smarty Pants Plugins” <> with a link to download the zip file. Then click “Install Now”.

After it says “Plugin installed successfully.”, click “Activate Plugin”.

Go to “Dashboard > Documents” and enter the License Key that was emailed to you and click “Save License Key”