Change a field in a different table by using a multiple choice option

I have a relationship, therefore, I have to select which record I want to show in Table A. I don't want to go and edit the related record and choose which record in Table B I just created. So a little background, I am creating a new record in table B (by clicking a button in table A) and I want the information from this created record to automatically update back to the related record in Table A. Table A will not see this until I select the brand new record from the multiple choice list.