How do I change the time to a 12-hour time if it's defaulted as a 24?

You search through the theme's *.rc files until you get to the part where the time is formatted, and change it to what you want. You'll need to figure out which module is displaying the time (likely xlabel) and open the help file to see how to change the time's format.

Many themes allow you to configure this through a specific theme management interface. Read your theme's documentation. If not, then follow what xcal said.