You can upgrade directly the trial version to the full version. Open any of the expiration reminder messages, click Upgrade and then follow the self-explained steps to install the product.
But if you you want to uninstall the trial version of Microsoft Office, try using Revo Uninstaller. Even if it can't uninstall the application normally, it will force it to uninstall by wiping out all traces of its presence from the system.
After that, try running AdwCleaner to make sure that there are no leftover rogue registry keys or toolbars in your system.