how

What to do before selling or giving away your iPhone, iPad, or iPod touch - Apple Support

I Think the only way that you can do this is by syncing with iTunes. I'm not sure that this still works this way, but there is no way to do it on the device itself.

Take a look at this.

How to delete Diagnostics and Usage Data On iPad Air?

You can look at this as well.

https://www.youtube.com/watch?v=SSbpbUIErUs