i currently have a pass which is due to auto renew on the 14th dec,i would like to add a 6 month pass,so do i wait till a few days before auto renew to cancel then add 6 month pass or cancel now and add 6 month pass?
No need to cancel, just go to your My Account > My Passes page
http://account.nowtv.com/passes
and click the apply a voucher link, your pass will then sit in your account and kick in when your current pass comes to an end. For future ref you can apply a new code whenever you have less than a month left on your current pass.
If you want it to carry straight on from your current pass, then don't wait, add it now. You won't miss out on any days as it won't kick in until the 14th but by adding it now you don't risk leaving it too late and getting charged.