Header customizing option seems to have disappeared…

I am trying desperately to change the image of the header…I applied an image once..it is there…but now I cannot find how I did it…under APPEARANCE…there is nothing with HEADER….all the videos etc I have have watched ( WP Tutorials and YOU TUBE) say to go to HEADER under APPEARANCE>…so how did i apply a header image..and now I cant find anything that changes it