My iPhone battery maximum capacity percentage went down

Apple states batery service life as 80% at up to 1000 cycles. We have old iOS devices that are well under 80% health and far of 1000 cycles, yet still produce acceptable runtimes before recharging..

Don't obsess over numbers. Use the phone as you need and, if the time between needing recharging is so short that the phone is hard to use, then get the battery replaced.

At 87% yours is not on the brink of death.

You can’t reverse the battery degradation. If you want to restore battery health, you need your battery replaced at an authorized Apple device center.