Like other Redmi or Xiaomi watches, you can set a PIN lock on the Redmi Watch 2 Lite, as shown below:
Step 1: Swipe up on the watch home screen and click the Settings icon.
Step 2: Scroll down and select the PIN option.
Step 3: Click on Set PIN and enter the PIN you want to set.
Step 4: Confirm the PIN again and it will be set.
Step 5: Now, you have to enter the PIN every time when the screen is off.
Step 6: To turn off the PIN, click on Turn off PIN and enter the PIN.
Source : wearablestouse.com