First upgrade to DSM 7.0 through Control Panel

Go to the Synology Download Center, Select product type and Select your Synology Product, then Select DSM version currently installed and the latest DSM version offered for this product as a target DSM version

Upload the downloaded update file in Control Panel –> Update & Restore –> Manual DSM Update and start the update

The update should run without issues, after which there are a few extensions which are obsolete and which you can uninstall