after IOS 14.5.1 update battery draining too fast what is matter ?

You will have to define “too fast”. And, as it’s an older phone, check your battery health in Settings/Battery.

Phone year old I’m having same issue after update drains. Not even getting a day

What model phone? What is the maximum capacity when you go to Settings/Battery/Battery Health? Are you charging the phone overnight, every night, as you should?