Which method should I use to update values of a custom field

Which method should I use to update values of a custom field

Hi, I want to know which API method should I use to change the values of a custom field. For my purpose defining the values of a custom field in the design it is not useful, because I want to change the adress at the moment of creating an Invoice depending from which account it came, as an account can have several adresses, the adress of the Invoice can change too so I think the solution is a custom function, but I don't know which API method could work. 

Hope you can help me, thanks