Hello sir my iphone xs yesterday battery health 96. or today 88 why i am so much tensions

You battery might be reaching the critical point of its life and can have unpredictable behavior (create a full backup of your iPhone as soon as you can).

If it continues to have this trend in the next few days, for your peace of mind, I suggest that you make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector. After checking it out, they'll give you all the available options.

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.

Mi phone X battery keeps reducing and I don’t know why. Moreover it’s on service mode