Trying to change ADT alarm code but no longer an ADT subscriber

My wall and electrical system were linked with an ADT control panel when I purchased my home a year ago. We still use the alarm itself in case someone tries to enter the house while we are asleep even though we no longer employ ADT. (We are working on installing SimpliSafe, but we are not quite done.) We want to alter the alarm code because our former flatmate is no longer welcome, but it does not appear to function when we follow the instructions we saw online. We are using the Safewatch Pro 3000 version of the control panel. I would be grateful for any guidance.