You can check your User ID by doing the following:
1. Tap More on the main menu bar.
2. Tap Settings
3. Tap Profile
4. The ID you have set will be displayed under User ID.
*The only way to change a User ID once set, is to delete your LINE account and start over. If you delete your account, all data such as your purchased stickers, phone number registered on LINE, Friends/Groups list, chat history and registrations with other integrated apps (such as LINE GAME and LINE PLAY) will be deleted permanently.