My iphone can't update ios 13.5.1. I don't know why

Go into Setting > General > Software Update --follow any on screen instructions

Here is a support article with more info: https://support.apple.com/en-us/HT204204

Make sure you are connected to a Wifi network, the download won't star on just a cellular signal.

Also please be sure to make a full backup of your phone before starting any updates.