Hello,
A staff member has replied to your question:
Hello Pang,
Thank you for contacting Steam Support.
We apologize for the delay.
Your PIN has been reset.
If you wish to create a new PIN, please restart Steam and log back into your account. Once this has been completed, you will be able to create a new PIN by doing the following:
1) Click "Steam" in the upper left hand corner and select "Settings”
2) Click "Manage Family Options..."
3) Select the options you would like to edit, remove or update using the Family Options Wizard
4) Once finished you will be asked to select a PIN, you may change the PIN or continue to use your existing PIN
Please let us know if you have any other questions.