ms access form update

hi i need urgent help to know how to update a form related to a table with a relation 1 to n to another table.
in the form there is client id and end user id sometimes the end user is the client by default. when entering new contract number i have to get automaticly the phone number adress... of the client and same for end user. so i need macros or anything else to update the informations in the tables related to the form , so that the update will be registered in the tables.
thanks