Q: I have too many pc's authorized.  How do I delete All first?  No button for that.

Use the Deauthorize All command documented in this article, or if it isn't an option, the 'contact iTunes Store support' link at the bottom of it.

(120850)

Use computer iTunes > Store > View Account > click the Deauthorise All button.

After that menu Store > Authorise This Computer.

Note: you can only do that once a year.

Are you using iTunes on Mac or Windows? and what version of iTunes?

This is how you do it on a Mac with iTunes:

How do I make iTunes deauthorize my old computers?

Good luck....