inadvertantly downloaded Yahoo app on my Iphone 5. How do I delete it ?

I'm guessing the same as any app -- touch and hold the app icon until apps start vibrating then touch the X in the corner of the app icon.

Deleting a 3rd party app should be in the user guide.

Press and hold on the app's icon or on any app icon near the app you want to delete until all icons begin to wiggle. Select the X on the app you want to delete. Press the home button when finished.

It is pretty well hidden and not in the section where it talks about arranging apps. Page 103 of the following manual: http://manuals.info.apple.com/MANUALS/1000/MA1565/en_US/iphone_user_guide.pdf

See page 103 of the user manual:

http://manuals.info.apple.com/MANUALS/1000/MA1565/en_US/iphone_user_guide.pdf

You may want to reveiw both the chapter on Basics, which includes some other information on using apps.

Best of luck.