Like when you’ve gotten a new sim, and the new sim comes with a new SIM pin that you just can’t remember.
Simply go to Settings > Security, where you find Set up sim card lock at the bottom.
I don’t recommend to disable the SIM card lock. But, you have a setting for it there.
To change SIM pin, simple tap Change SIM pin and just follow the guide. Make sure that you DO remember the new SIM pin, or have your PUK (Personal Unblocking Key) ready. It’s often printed on the main sim card, where you press out the sim you use from.