Changing the headers

Can someone please tell me how you can change the From: header in the email? I know it's possible... my computer teacher used to do it all the time... but how? (unfortunatelly his explanation was to hard to comprehend for me at the time). Thanks a lot... And in case it helps... I'm using a VAX.. regular text account. If it makes any difference. Doesn't it have something to do woth getting into the mail port? Roki