Yeah, I realized I was using very low data per month, around 4-6GB a month only because I am connected to WiFi a lot, plus my carrier has a data roll over feature, so if I didn't use part of my data last month, it gets added to the next month's allowance. So I decided paying for something bigger than 6GB is just wasted money.
Before you downgrade, as stated above, take a look at what your carrier offers and then your average usage of data, and your max usage. So you should try to estimate how much you should reduce what you're paying for. Compute how much you think you need, then add maybe 500mb or 1GB over that for safety.