Viewfinder is dirty - how do I clean it ? Nikon film camera.

Hi I have this old Nikon camera that my dad got like 30 years ago maybe and never used.I dusted it off and wanted to see what a film camera is like.The viewfinder is pretty dirty - how do you clean it?https://ibb.co/jhrb1fK https://ibb.co/tJg0pry https://ibb.co/RTmSg1B via /r/photography https://ift.tt/2IBDNsw