how can i delete one email in a stack of them?

Are you talking about individual emails or a conversation/chat where you reply and then they reply back? It they are individual ones just hitting delete should do it, but it sounds more like is send,reply,reply back, etc. All based on the original one sent.
If that is the case and you were the last one to reply then it doesn't matter if you delete the whole stack because if they reply to you they will all show up again. If you just want to keep one of them then it would be better to either pring that one only or copy/paste the text you want to keep into a word processing program and then delete the whole stack.

In gmail, each message has a "reply" place at the top right of the message. Beside that reply button, there's a down arrow that will pull up a menu that includes "delete this message".. that will remove just the one you want to remove... IF you open that particular mail first and use its own arrow. Just tried it in a stack and it removed just the one I had opened.