Need to Change Sender In EMail Template--How Do I Do This?

Need to Change Sender In EMail Template--How Do I Do This?

If possible, I need to change the From address in a couple of email templates I've set up.  The address that is currently there has a pulldown arrow next to it, which generates the following options:

Current User's EMail
Record Owner's EMail


However, I can't seem to find a way to enter an address that is different from the default address.  Is it possible to change the default address to something else?  Thanks for any help!