Hi Larry, you say you have the Yahoo app on your iPhone and iPad?.
If this is the case, have a look at the screen shot here.
After you select the first email using the tick box, a new tick box should appear (indicated by the arrow). This tick box allows you to select ALL of the emails in the current folder (inbox etc).
Simply select them all, and go to the bottom of the selection where you can now use the 'trash' button to delete them all.
And try this to delete ALL emails using the Apple mail app (taken from one of my posts in a previous thread)
"There used to be a method to remove all emails from an account in 'mail' using the 'delete all' command.
Unfortunately Apple seem to have removed this facility for multiple emails in iOS 10, and you can now only delete one email at a time. This is fine for a few emails, but I wanted to remove approx. 6,500 !.
However I have since found that there is a hidden method in iOS to easily remove all emails in Inbox or Draft. And it needs only a few clicks, works on any iPhone iPad in all versions of iOS 10 (including iPhone 7 and iPhone 7 Plus.).
The solution is moving all emails to Trash, which still has a 'Delete All' button.
Step 1. Go to Inbox or other account mailboxes that have no "Delete All", and click Edit on the top corner.
Step 2. After selecting one email, press and hold the "Move" on the bottom and then uncheck the email you selected. (Make sure not release "Move" until you uncheck all the items you checked before.)
Step 3. It may take a while, depending on how many emails are in the account, but it will then bring you to a page that allows you to move all emails to another folder, such as Junk and Trash. In this way, you can move all emails to Trash.
Step 4. After moving all emails to Trash, just click Edit > "Delete All" in the trash to get rid of all emails from Inbox or other boxes that have no "Delete All".
Hope this helps
Steve"