If you're an existing customer you need to go to the product page of the tarrif you want to switch to. Sign in to your account then go to:
Towards the bottom there's an option for existing customers to sign up.
Only takes a minute and then you should be moved over at midnight the next day.
Thanks - very helpful! After doing this it's necessary to go to the account dashboard. It will tell you that everything is ready for the switch (i.e. you already have a smart meter) and it's necessary to click the button to confirm the switch.
The switch happens at midnight. I clicked the link above and confirmed on the dashboard at around 11.50pm and now, a couple of minutes past midnight, the dashboard shows that the current tarriff is Go.