Where does the business unit field value on a cash or unapplied receipt come from?

Question:

When we enter unapplied cash, the business unit defaults to "1" so that when running an AR report by business unit they all fall under the "1" business unit. 

A) Is there a way to change the business unit on existing items that are in unapplied cash? 

Also, is there a setting that would change what the default business unit is when the receipt is entered--so that it's populated with the company number instead of "1"?