Address field: Set Address via Apex

You set the individual fields, such as BillingStreet, BillingCity, BillingState, BillingPostalCode, and BillingCountry (plus the geolocation codes, as well, if you prefer). The address complex field is a convenience field derived from the parts, just as a contact or lead's "Name" field is really a concatenation of the various parts of their name (first, last, etc).