According to the Now Broadband website, it is possible to upgrade your broadband package if you currently have NOW Brilliant Broadband or NOW Fab Fibre, but if you already have the fastest package (NOW Super Fibre), there is currently no upgrade path available. The website also notes that prices may change during the contract period.
To upgrade or downgrade your Now Broadband package, you can contact the NowTV Broadband team. The upgrade or downgrade is subject to availability at your postcode and the price increase.
There may be no fee to upgrade or downgrade your package, but your new speed and new price adjustment will take effect on the day of your next bill.