I assume you are talking about manipulating an active ArcMap session from outside the application boundary. To do this you need to use
IAppROT to get a reference to the specific ArcMap application instance you want to manipulate. From there you can access its document, active view, map, etc. and manipulate them as needed (see @artwork21's answer).
Just be aware that any reference type objects that you create, especially singletons like factories, you must use
IObjectFactory to create them within the ArcGIS application's process.
The Activate ArcCatalog from ArcMap sample shows how to use IAppROT to obtain a reference to ArcCatalog. You would need only to change it to look for ArcMap (
IMxApplication) instead of ArcCatalog (
IGxApplication) and remove the logic that starts ArcCatalog and waits for it to be added to the AppROT if that is not your intention.
You will likely also need to check out a license using
IAoInitialize before creating the AppROT object.
Also worth noting, as an alternative to using VBA in Access, you could use ArcObjects in Python, and call the Python script from a macro in Access.