I can never find a link from the Control Pages to the Sales page, so I just go direct to
https://www.s-s-l.co.uk/orderpage.cgi
Then click on the line that contains my address and select "Change your tarrif".
It's a bit fiddly TBH and more complicated that it should be ao I've prodded RevK on IRC to see if he's willing to create a dedicated tarrif change option.