I want to update iOS 13.5.1

To update to the most recent, compatible version of iOS, please see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

There is no Apple device capable of updating to iOS 13.5.1.

What device do you have?

What is the existing version of iOS?