how to update apple credentials

Two Comcast techs were here to replace modem. They managed to screw up my access to Apple Mail.

I cannot send mail and if I try, a popup advise that I cannot connect to the account comcast.net. I am asked to enter the password for my Comcast user name and after I winter a correct password, Comcast tells me that it cannot verify account name or password. As an aside, Comcast does manage to deposit the charges I pay each month.

Comcast tech help write that I must update my credentials in Apple Mail to continue sending from Apple Mail. I have no idea how to do that. HELP!!

Comcast techs also write that a third part email client is being used to access my Ccmcast mail???