How can I get an iPhone battery replacement? Maximum capacity at 70%

Rechargeable batteries don’t last for ever. They naturally lose their ability to hold a charge with use and time.

Make an appointment at the Genius Bar of your local Apple Store and get the battery replaced. It’s not that expensive.

On average, the battery will drop 1% per month. So for a 2 year old phone with a drop of 1% per month or 24% total, the battery health would be 76%. There does not appear to be anything wrong with your phone. Get an appointment at an Apple Store to get a new battery.

zeinab184 wrote:

Is it something safe to change the battery or it will be hard for me to sell the phone after changing the battery

and btw the battery was 71% a week ago today it’s 69% and I really don’t use it that much

The only way you could make it hard to sell or trade the phone would be to let ANYONE other than Apple or an actual Apple Authorized Service Provider replace the battery. As long as you have Apple replace the battery, you will have an authentic Apple Battery in your phone and you won't have an issue selling or trading in the phone.