How can I change the date and time format on my MacBook Air?

Two days after upgrading my MacBook Air to version 12.7.6, a host of problems occurred. I guess it’s appropriate to post separately about each one. I've got a Macbook Air, Early 2015.

I can’t format the date and time at all. It’s stuck on what I guess is the default format: abbreviated day of the week, and time of day. I want the date and the time including seconds. I know where in Settings to make the change but nothing I do works.

Can anyone help?