Is the passcode preventing access to your entire phone, or is it a passcode just to use the Square POS app on your phone?

Assuming it's on the app, you should be able to change access codes under the Team section in the Dashboard by logging in on a computer browser. This assumes you have owner/administrator permissions to log in to the Dashboard.

If they've put a passcode on the entire phone itself, that's beyond the scope of this forum. But in general you should be able to log in to your phone account from a computer browser to remove such a block. You may have to do a reset of the phone though.