The steps below outline how to connect to the Arduino YUN using SSH.

First, you'll want to make sure that your Arduino YUN is powered and connected to the same network as the machine you will be connecting to it with via wired ethernet or WIFI.

Next, open up terminal and run the following command

ssh root@arduino.local  

If this is the first time connecting, it will ask you authenticate by answering yes

The authenticity of host 'arduino.local (192.168.1.226)' can't be established.  
RSA key fingerprint is 2a:91:81:3c:5e:c7:ce:8c:bf:3e:c4:e2:69:eb:bc:3d.  
Are you sure you want to continue connecting (yes/no)? yes  

Next, it will ask for a password. The default password is arduino

After entering the password, you will be connected to your Arduino YUN via SSH.

Arduino YUN SSH connection in terminal