@Anonymous User
Welcome to the forum.
Just go to your My Account > My Passes page
http://account.nowtv.com/passes
and click the apply a voucher link in the Sky Cinema Pass section and follow the steps to enter the code. You'll then get the next 2 months movies free, just paying £6.99 for your entertainment pass. Then after your 2 free months you'll go back to paying £17.99 for both again.
If you buy both passes every month, you might be interested to know you can buy vouchers for NOW TV from Game (online and emailed immediately or in-store), Argos, Currys and PC World which can save you money in the long term Prices are
Entertainment - 3 months - £15
Entertainment - 5 months - £25
Movies - 2 months - £15
Movies - 3 months - £25
Sky Sports Week Pass - £10
Some people also like to buy the NOW TV boxes with bundles of entertainment or movies vouchers when they come on offer in places like Currys, Argos and Amazon and again the vouchers in those can be used by existing customers.
@Anonymous User
It's not actually a direct debit but a recurring card payment, so again you go to My Account > My Passes and click the cancel option next to the pass you've got. Best to do it plenty of days before your pass ends so you don't forget (you don't lose any days by cancelling early).
You don't need to cancel if you are planning to add another voucher straight away, just enter the voucher when you are in the last month of your current one (and before it ends otherwise you'll be charged) and it will kick in automatically when the current one ends.
Just remember each time time you add a voucher it automatically sets it up to renew at the end, so if you don't plan on carrying on after the voucher ends remember to cancel it again.