You can't edit your birth date after you've saved it.
What you can do is delete your account and create a new one with the correct date. Before you do that, make sure you have anti-theft deactivated and are signed out of your account.
1. Deactivate anti-theft:
Go Settings > Account > Anti-theft. Select "off" to deactivate Anti-theft, then enter your password to confirm.
2. Sign out of KaiOS on your phone:
Go to Settings > Account > KaiOS account > Options > Sign out, and enter your password to confirm.
3. Delete your KaiOS account:
Go to your account page.
Enter your email address and password and click Sign In.
Go to Edit account > Delete account.
4. Create a new KaiOS account
Go to Settings > Account > KaiOS account > Create account.