I know I have done this before but I don't remember how or even know how i undid it?

If your using an email client (outlook, outlook express, thunderbird, etc). Open up the email client. Click on Tools/Account Settings (might be called something different in each program). Find the account you want to set as default, click on it to highlight it and select "set as default". Depending on the email program you use, you might just see a link when you click on the account settings for the account you want that says "select as default".