Radio Updates: Spanish stations KTNQ and KLVE (107.5-FM) have rescheduled their postponed Cinco de Mayo celebration for May 24 at the Los Angeles Memorial Coliseum. The celebration was to have been May 3 but was canceled because of unrest in the city. . . . Talk station KABC's 19-hour live radiothon "Spotlight on AIDS '92: The Facts, the Fiction, the Future" has been rescheduled for June 24. Originally announced for May 7, it was held up because of the local strife. . . . KABC also is sponsoring an essay contest for junior high and high school students in Los Angeles and Orange counties on "How to Save My City."