When you start a new game, a screen will pop up after the Plane Crash cutscene for you to choose the sex of your character.
If you have already passed that part and still wish to switch your character's sex, then you can tap the arrow button on the right side of the screen and then tap "Contact Us", and our Customer Support team would be happy to switch it for you. You can also join the LOST Official Chat Group on Discord by tapping the following link: https://discord.gg/7YYJ8cq and then contact the admin there to switch the sex of your character.