OK, say you've got tblAddressFix with fields:
AddressID - Autonumber (key)
MyAddress - Text
MyState - Text
In this example, It's populated with two records--field [MyAddress] contains (1) "New Orleans, LA" and (2) "Los Angeles, CA"
The intent is to place the state abbreviation in field [MyState], and remove it from field [MyAddress].
Use an Update query. The InStr() function will allow you to determine the position of the delimiting comma-left of the comma is the city, right of the comma is the state, i.e.:
UPDATE tblAddressFix SET tblAddressFix.MyAddress = Left([MyAddress],InStr([MyAddress],",")-1), tblAddressFix.MyState = Trim(Mid([MyAddress],InStr([MyAddress],",")+1));
This can all be done from the query grid, starting with a Select query, then transforming it into an Update query and adding the code to the Update To blocks.