How can I downgrading iCloud backup from 2TB to 250GB on iPhone?

My iphone always shows prompt messages from icloud that I really need to update and upgrade it. It also shows that it's been 12 week since my phone files has been last synchonize due to lack of icloud back-up space. But the I really feel that my payment for a2TB upgrade is too much for me. Thats why I'm asking how to upgrade icloud backup lower than 2TB. Because it doesn't show it as an options on my iphone.

[Re-Titled by Moderator]

Original Title: How can I avail the 250GB icloud backup rather than the 2TB icloud back-up because I think 250GB is enough for me, but the options for my icloud backup updates only shows 2TB or more