how to regain the battery capacity?

Batteries are degradable products. You cannot regain battery capacity. All you can do is when the battery health drops below 80% is to bring your phone in for service

See this article for further reference]About battery health on your iPhone - Apple Community

You replace the battery when it wears out. All chemical batteries loose capacity over time. That capacity cannot be regained as it is a consequence of degradation of the chemicals in the battery.

Batteries have finite useful lives. When they loose enough capacity they cannot meet your needs any longer, you replace them.