Today, I've dug up the answer to that and it's an official one from Netflix itself. I have also take the pain to list out steps with screenshots. First, here's the link to the official answer: Netflix Data Consumption Setting. (Note that this can only be done from your computer, and not mobile devices. But the settings will apply to all your devices, as it's an account level setting).
How can I control how much data Netflix uses?
Watching movies or TV shows on Netflix uses about 1 GB of data per hour for each stream of standard definition video, and up to 3 GB per hour for each stream of HD video. This can create headaches for Netflix members who have a monthly bandwidth or data cap on their Internet service. Below, you'll find a few ways to reduce the amount of data Netflix uses, without having to resort to drastic measures (like actually watching less Netflix).
Adjust your data usage settings
Adjusting the data usage settings for your account is the easiest way to reduce the amount of bandwidth used while watching Netflix. There are four data usage settings to choose from. Each estimate below is per stream:
Low (0.3 GB per hour)
Medium (SD: 0.7 GB per hour)
High (Best video quality, up to 3 GB per hour for HD and 7 GB per hour for Ultra HD)
Auto (Adjusts automatically to deliver the highest possible quality, based on your current Internet connection speed)
To select a setting that works best for your Internet plan, navigate to the Your Account page and select Playback settings in the Your Profile section. It can take up to 8 hours for these changes to take effect. Restricting data usage may effect video quality while watching Netflix.
Data usage settings only apply to the Netflix profile you are in when you set them, meaning you can have different data usage settings for each profile. If you are concerned about the total amount of data that Netflix uses, make sure to change this setting for each profile.
And here are the illustrated steps as I make the settings change for my own account.
I go to my account page, always accessible at https://movies.netflix.com/YourAccount
When it loads, I click on Playback settings.
I set it to the data consumption level I am comfortable with.
You can choose low but I chose Medium because I already use the keyboard combo and hidden menu I talked about in the previous post. That one gives me way more data consumption options and I when I am on free wifi I can go as high as I want.
Don't forget to save your new settings.
And that is it. Just two easy steps.
Enjoy your Netflix subscription! I have already watched about 5 movies already. In fact, I am canceling my subscription as this is unproductive (taking me away from more productive use of my time).
Here's how I cancel my subscription (luckily, it will take effect only after the free month subs).
On same my account page, I click on Cancel Membership.
Then I confirm the cancellation.
I get a notification that it has been effected.
And that's it! But I still get to watch movies till Feb 6, 2016.