There are two things you might try - I'm not sure which one you'll have to do because I've not tried to change my displayed name:

You can change the name associated with your Yahoo ID. You should be able to go to the group page (or the Yahoo group page for any group you're in) and click on My Account up at the top. Click the "Edit" button to the right of Address/Contact Information, and it will let you change your name.

However, I'm not sure that's where it gets your name that it associates with your posts. That's probably where it gets it if you are sending your post from the Yahoo@Groups site. If you are sending from your email, I think the name that shows up really comes from your email program that you use for whatever kind of email you have. Since it looks like you have Yahoo email, you should be able to change that somewhere in your Yahoo email setup.

Somewhere in your mail setup, there is a place where you get to determine the name that's sent with your email. This would be the same whether you send it to a Yahoo group or anyone else. I'm not familiar with Yahoo email specifically but that's where you need to look for how to change it.

Kitty