You can update your existing installation retain your torrent cache and DHT index by copying the files from a newer release asset on top of the existing files. Then run cove from the same location as you usually do. Your existing DHT index and torrent cache files will be reused.
If your cove install is out of date, cove should log a message to the terminal when it starts up. You can also check the releases page to see if there is a newer version available.