Here are the steps for a rolling upgrade as outlined in the product's Best Practices page:
Open the SOSS management console on the host, select the local host from the list on the left, and go to the "Host Status" tab.
Click "Leave" to cause the host to leave the store.
Wait for the host to completely finish leaving. Its icon will change from green to yellow as it sheds load and will finally become red once it successfully leaves the store.
Uninstall SOSS on this host.
Install the new version of SOSS on this host.
Open the SOSS management console and configure the host as necessary. Note that the host should automatically update its license key entry from other SOSS hosts when the SOSS service process restarts and detects an existing SOSS store.
In the SOSS console, go to the "Host Status" tab and click the "Join" command to join the store.
Wait for the host to finish joining. Its icon will change from red to yellow as it is joining, and will finally become green once it successfully joins the store.
Verify that load balancing has completed and that the distributed cache is running normally before upgrading the next host.
Please note that the Rolling Upgrade procedure may only be performed within the same major version of the product. Please see the Release Notes for the version you wish to install to verify it supports a rolling upgrade from the existing version of SOSS running in your environment.