Begin by opening your account, and under the Your Profile section, find the Playback Settings. If you’ve got the right plan, next in the order of operations is to tweak Netflix’s streaming options. You will need a Premium plan if you want to watch in 4K UHD, which currently costs $18 per month. If you aren’t sure, we’ve got a breakdown of all the plans the streaming titan offers.
Your plan most likely supports HD streaming, but some plans allow for streaming only in standard definition, and 4K UHD streaming is available only on the most expensive plan, so it’s worth at least double-checking. Your first step should be to check your Netflix plan and settings.