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.