How do i go about deleting large amounts emails in yahoo mobile all at once?

If you want to mass delete mail you have to be using full featured mail. Basic Mail only lets you do a page at a time. Switch to full featured by clicking on the link at the top right of the Basic Mail page and then you can delete as many as you like at one time. Just check the box at the top left of the message list to select all and then hit delete.

i don't have yahoo mobile but what i do is
click the check box of an email, if you have, let's say 20 emails down the list that you want to delete hold Shift and click that check box of the 20th email. That should check all of the check boxes of only those emails. Then click Delete