After updating my iPhone to iOS 15.6.1, my battery drains so fast

Have you checked your battery health recently? Apple only considers that a battery requires replacement if it cannot hold, at least, an 80% charge after 500 full recharging cycles.

Also, for a while after updating an iPhone, it may go through a series of activities before it "settles down" to a more "normal" battery consumption rate ... not just background app updates.

Ref: About the battery and performance of iPhone 11 and later - Apple Support

that’s funny you should mention that I updated mine too I woke up watching the news at 4:30 AM they before yesterday two days ago and the first thing they started with was anyone who has an iPhone iPad I iPhone anything must update it immediately or you’re taking a risk of and all iPhones being hacked the thing is I am mediately updated the software but it didn’t install it until like two days later so that was really weird but my battery drains even when I lower the the lighting the background lighting and I don’t use my phone hardly other than answer calls text my daughter or I get on YouTube or watch a movie sometimes Oh and check my indeed account off anon and yesterday it drained down to 2% within four hours if that

I have checked all of my settings, apps running in the background, network reset settings everything. My battery goes from 100 down to 95 very quickly. Let's use reasonable deduction. Before the update, my phone drain was minimal. After the update, my phone now drains fast. Reasonable deduction says that there's something in the update that Apple needs to address immediately. As I am not spending a thousand dollars on a new phone.

What exactly do you consider "so fast?" Which exact version is your iPhone? Is the battery in this phone the original one that it came with?