Jason,
Sure you can do it. Go to Admin panel and use language and translation tool. Search for the phrase and rename it to whatever you like.
But in your case don't rename the City field. This field is very important and is used by other modules as well. What you need is another field.
Go to admin panel -> Record Browser. Open Companies recordset and ADD new field, text type, not required, visible in table view and call it Customer Number.