How to get back my iPhone battery capacity to 100%

Batteries are consumable devices and degrade over time. So long as you are above 80 percent, there is nothing that you need to be concerned with. Learn more https://support.apple.com/en-us/HT210557

To get your battery back up you would have to have a battery replacement from apple but there is nothing to worry about as long as it is over 80% battery health