There are a few ways to tackle this so relax in the knowledge something will work for you. I’m with Timo, I’d use CSS selectors for this (that’s what browsers do natively, so why not?)
First, take a read of this article I posted a while back. There’s a section devoted to dealing with iframes. Get that knowledge under your belt and then post back if you still have questions/problems.
And just for completeness, you might also benefit from a related article: DevTools