How do you get your battery back to 100%

You can't get it back to 100% as you have been using it already. You can make the battery health better by following these -

• Stay on the latest iOS upgrade. Every major upgrade comes with new features to preserve and extend battery life.
• Try to keep your phone in temperatures between 16 and 22 degrees celsius on average. Anything above or beyond might be damaging to your lithium battery.
• Avoid excess heat when charging. Some cases or surfaces can cause your phone to heat up, which can damage the battery. Check it while it is charging to make sure it stays cool.
• Store your phone half-charged. Emptying the battery completely could cause it to fall into deep discharge, which will cause significant damage. The drainage from a full battery could have the same effect.
• Turn on auto-brightness. The brightest screen settings tend to drain your battery quickly, which forces you to go through charge cycles quickly and lose battery health.
• Check your background activity in Settings > General > Background Refresh. Too many apps running in the background will drain your battery.