After entering the Yostream Studio, you'll see the following screen.
Click Stream with Yostream.
Under the General tab of Settings, you'll see the default title under Session info. Below the title field, you can see another field where you can add your stream's description.
Update the title and description. After filling in both fields, click Update session info at the bottom.
Now, close Settings and you can see the updated title. The description is not visible on the screen here; it is only displayed on the platforms where your stream is being broadcast.