If this is your first deployment, you will only see a button to proceed. Otherwise, you will see previously deployed versions and decide with how much traffic you'd like your new deployment to be published.
sk_variantwith the id of the displayed deployment. If you need to manually change the version, simply overwrite the cookie value with the id of the other deployment. You can find the id in the address bar when you click on a deployment. Keep in mind that this deployment needs to be published, otherwise Stormkit won't recognise the cookie value and will display one of the published versions.
As this operation is completely done on the load-balancer level the user won't see any flicker effect which client-side split testing libraries usually cause.