You can use STR_TO_DATE(str,format)
function to convert time format:
SELECT STR_TO_DATE(`field_name`, '%h:%i %p')
FROM `table_name`
# this will convert '7:30 AM' to '07:30:00'
If whole DATETIME format is outputed, you can use TIME(expr)
function to get only TIME part.
Demo (don't know why sqlfiddle is ignoring TIME() function; it works on localhost).