Once you’re signed into your Smashcast account, go to the Profile > Connections page
If you want to disconnect your Smashcast account from YouTube you’ll need to go to your Google Account Settings (easiest way to do this, open your Gmail inbox, select your account in the top-right and click My Account), under ‘Sign-in & security’ there is the option for ‘Connected apps & sites’. Select ‘Manage Apps’ and find the ‘Smashcast YouTube Upload’ option. From there you can remove access to YouTube from Smashcast.
Here you’ll see all the options of what you can connect Smashcast to
Select the “Connect” button beside YouTube and follow the prompts in the pop-up window
(As of September 19, 2016)