Don't worry, you only need to install Nextcloud client via Terminal - future updates will be handled graphically by the elementary OS AppCenter.
We are now ready to install the client. To do so, enter the following commands in your Terminal:
sudo apt install nextcloud-client
And that's it. You successfully installed Nextcloud client on elementary OS. Start it up and configure it. Once you fully configured it, you'll fall in love with its integration into elementary's File Manager.