I change my battery but I received a message it says unknown part what should I do.

A genuine Apple replacement battery was not installed on the phone and instead, a "knock off" was used. You only get an Apple battery at the Apple Store or an authorized Apple Service shop, so you know where to go to get things corrected.

The message will continue to appear until a genuine Apple battery is installed.

In addition, a non-authorized part may not allow the Battery Health settings to display accurate information about your phone like Maximum Capacity.