How do I do that? I clicked eM Client, then Manage, and was unsure what to do after that>
I believe you have Windows 10 from your previous posts. See @sunriseal post for another explanation how to set the defaults to be eM Client.
Also, after setting eM Client as default, does all email go thru Em Client?
As you use IMAP accounts from you’re previous posts, all your various mail programs “will see the same email no matter what mail program you use”. So just make sure they are all setup as IMAP accounts.