How to get back my iPhone battery capacity to 100%

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

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