In Ubuntu 17.10 and later how do I change the clock to a 12 hour format?
Solution 1:
Click on Activities and search for "Settings" and launch it. Click on Details at the bottom of the sidebar and then select "Date & Time".
You'll find an option to change the time format.
For an option with more customisations, you may use a GNOME shell extension called "Clock Override" as mentioned in my answer here.
Solution 2:
In search bar, search for "Time" and the "Date & Time" setting comes up, click on it and change to "AM/PM".