Hi Agnes,
Outside of going to an Apple service provider, the easiest way I've found to do this is to get a screen/battery reprogrammer from Qian Li or JC, remove the original battery, (Make sure to follow the proper guides) connect it to the programmer, press read, then connect the new battery to the programmer and press write. Once you reassemble the phone, the battery health should be reset. Now, this isn't a foolproof solution, as Apple is trying to prevent you from doing this. Sometimes it doesn't work at all, sometimes it doesn't reset it and gives you the old percentage. This is just the best solution I know of.
Hope this helps!
-Cam
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