How to change my password in mail?

Yes, I have a Mid 2010 desktop iMAC computer; OS X El Capitan 10.11.6; 3.06 GHz Intel Core i3; 4 GB 1333 MHz DDR3; Macintosh HD with 335.23 GB free 0f 499.25 GB. I'm now using Google Chrome, mainly because Safari kept beach balling on me. I don't use apple mail much, instead I use comcast mail. But now I can't get send email in apple mail any longer. I keep getting Can't connect to the account, "comcast.net". I have put in all of the updated & past passwords I know of, with none of them working. Is there some way of changing this password on my own? Frustrated. I read in Apple help where they tell you to go to advanced in Internet Accounts and this can be done, but the drop down window in internet accounts does not allow you to do anything until you put in the needed password.