I just booked a ticket through Airasia and I noticed that there is a mistake. I filled Mr. instead of Ms. Do I need to change that? If yes, please tell me how.
The wrong Gender shouldn't be an issue. As long as your name and surname maches the one in your passport, you should be fine. Some Airlines such as AirAsia even change it for free as seen here on SE.
Just to be sure, you could either dress manly or more seriously, just give the airline a call or write them on Twitter.