How do I change the time format from 24-hour format to 12-hour format, so that, for instance, the time shows as 1:00 pm instead of 13:00?

You’ll need to set it for your OS, since Lightning uses the OS’s time format. See [https://wiki.mozilla.org/Calendar:24_Hour_Format here]. Also see [/questions/1081279 this] and [/questions/996771 this] question.