HTC One M8 for Windows - Manual (Page 184)

Security
Assigning a PIN to help protect your SIM card
You can help protect your SIM card by assigning a PIN (personal
identification number) that you enter before accessing your SIM.
1. Swipe down from the top edge of the screen with one finger.
2. Tap All settings.
3. Swipe to the applications screen, and then tap phone.
4. Tap the SIM security On/Off switch to turn it on.
5. When you're asked to enter the default SIM card pin, enter 1111,
and then tap enter.
6. To change the SIM card PIN, tap Change SIM PIN, and then enter a
new PIN.
184 Security