Online or Mobile App
It's possible to change your PIN using your bank's mobile app or online banking website. This feature is often located in the customer service or self-serve area. Typically, you'll need to log in to your account and specify that you want to make a change, then follow the prompts to do so. Changing your PIN online or with a mobile app requires you to remember your current PIN, so be prepared to provide it.
Sometimes, when changing your PIN over the phone, you will speak to a person at your bank. Other times, you may find yourself changing your PIN using an automated system. If you know your old PIN, the automated system will generally allow you to change your PIN on the spot. If you don't know your PIN, or if you speak to a representative, the bank will likely opt to mail your new PIN to the address they have on file for you.
At the Bank
Many banks will let you bring your debit card into the branch to change your PIN using a simple debit card programming machine. One advantage of this method is that it allows you to make the change on the spot. Plus, you get to pick the PIN you want. While it's easiest to do this if you know your current PIN, going to the bank in person is also your best option if you have forgotten your PIN. Bring identification, and once bank teller verifies your identity, they will allow you to change the PIN.
At an ATM
Many banks allow you to change your PIN at any ATM. This method is convenient, since you don't need to go into the branch or navigate your way through the website to make the change. You'll need your current PIN to do it, but the ATM can program your new number immediately. The option to change your PIN is most likely going to be found under a sub-menu with the name "More Options" or "More," and not in the main ATM menu.