I want to change/update custom fields based on the changes made to another custom field

Summary:

I want to change./update custom fields based on the changes made to another custom field

Content (please ensure you mask any confidential information):

I want to create a script that Updates a field once another field has been changed. For instance I have custom fields Defendant and Department which I created in app composer. Once the Defendant field has been changed or updated I want the Department to visibly change instantly depending on the PrimaryPointofContact entered in the Department field. The Department field should also produce the Business Unit Name associated with the Defendant. I have tried using the following script but can't seem to get it working. Should I be using the BeforeUpdateTrigger