how do I clean the retina screen

You can search Apple support pages.

Generally, a dry microfiber cloth (that came with your iPhone) is the best. Please don't use any chemicals, as the screen has various coatings.

Cleaning Apple products:

https://support.apple.com/en-gb/ht204172

https://support.apple.com/en-gb/HT201294

and this:

http://www.macworld.co.uk/how-to/mac/how-professionally-clean-mac-iphone-screens -3609155/