How can I improve my iPhone's battery life?

First, go to Settings/Battery/Battery Health and see if there is a message about battery condition, and verify that the max capacity is above 80%. If there is a problem arrange to have the battery replaced: iPhone Battery Replacement - Official Apple Support

If the battery is OK, then you should be charging overnight, every night, with Optimized Battery Charging enabled. If the phone doesn’t go below 20% before the next nightly charging it is working normally.

Beyond that, review these tips: If the battery in your iPhone drains too quickly - Apple Support

Are you talking about daily use time or the longevity of the battery?

For any iPhone 13 model that has seen normal use, I would expect your Battery will need to be replaced by now. Once the Battery Health drops below 80%, it will be time for the replacement and you will also see decreased daily use time when the Battery Health drops as well.

Please don't obsess over "Health." One metric used to calculate health is not based on your battery's actual baseline capacity but rather on an average baseline or "design capacity" of all batteries of that model. We all know what "average" means:

My iPhone SE 2020 started at 102% health; averages say that means some poor soul got a phone that started at 98%.

I find battery "health" non-linear as well. In my iPhone SE, my iPad, my wife's iPad, and an older Macbook Pro, that the rate of health drop slows in the mid-80% range. My phone is 3 years, 7 months old and health has been in the mid-80% range for a year.

Apple expects the iPhone 13 battery to be good for 500 cycles†, but I find cycle count an inconsistent indicator. We have Apple devices with cycle counts well over 1000 that still give useable runtimes.

† — source: MacTracker database, free in the Apple App stores