Tuple requires all participants to be on the latest version of the app to start a call. If you try to start a call on an outdated version, you will be prompted to upgrade, which is a simple one-click process. If your pair hasn't upgrade yet, they will be prompted to upgrade and you will see a message saying that your pair needs to upgrade first.
When upgrading, you have the option to upgrade automatically in the future, which will happen silently when updates are shipped.
You can trigger a manual upgrade by clicking on your avatar to bring up the menu and selecting "Check for updates":
The current version of your client will be displayed to the right of "Check for updates".
You can also view the current version for your Tuple client by selecting "About Tuple" from this menu.
If Tuple will not open and you need to check your installed version, you can right click on the Application icon in a finder window, select Get Info, and then look for the Version number.