Same number of lines, less variables, no string comparisons. One atoi though.

int main() {

char time[11] = {0};
scanf("%s", time);
int n_hour = atoi(time);
char plus12 = n_hour >= 12;
n_hour += time[8] == 'P' ? (plus12 ? 0 : 12) : (plus12 ? -12 : 0);
printf("%02d:%.*s:%.*s", n_hour, 2, time + 3, 2, time + 6);
return 0;

}