Help with iMail

Received my new iMac Friday and have been trying to get my head around this thing. Been in IT for 25+ years so I have some experience and apologize if this information is someplace but have been unable to find it.

I was able to get my Outlook PST file imported to Mail by way of Thunderbird. The problem I have is that every message that is now showing in that folder as unread. This wouldn't be a problem except that there are a few thousand emails, in I don't know how many folders and subfolders. Is there a way to mark all of these emails as read without having to go to each individual folder and subfolder and selecting all emails in that folder and marking them read?

I have the same question for removing the "organize by conversation" under view without having to change each folder and subfolder individually.

Thanks for any assistance, Joe.
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2012 27" iMac, i7 3.4GHz, 24 GB RAM; 3TB Fusion HD | 2005 Mac Mini G4/1.25 GHz; iPad 16gb 1st Gen | Nikon D5100, 18-55mm f/3.5-5.6, 18-200mm f/3.5-5.6

Under EDIT, do Select All. Now under Messages select Mark and choose As Read. Not sure what you are trying to do with the Organize by Conversation. It just checks and unchecks to select whether or not the mail is organized that way.

Thank you, that worked. What I want to do is remove the group by conversation for all folders. I prefer to see each individual email. About halfway through with each folder so it looks like I'll continue like that.
Thanks again.