My iphonex is still in warranty but my back screen got damaged.

The warranty does not cover accidental damage.

However, if you purchased AppleCare+, that does cover 2 incidents of accidental damage for a service fee.

If you purchased AppleCare + with your iPhone X then you get two instances of accidental damage covered for a deductible fee. If not your warranty does not cover that and you will need to do an out-of-warranty replacement.

It will be $99 if you have AppleCare +, $549 if you do not.

iPhone Repair - Official Apple Support