Generaly (but not always) a subform is related (linked) to data within the main Form of where is resides. By changing the subform control Link Child Fields and the Link Master Fields properties, then a subform Requery (all via code) in the OnClick event of your ComboBox will usually do the trick. It all depends on what you are really trying to accomplish. Inserting data within the subform can also be accomplished providing the privileges are available.
To receive better results from your post, it may be a good idea to provide a small sample DB of what you are trying to accomplish and allow members get a good grasp of your needs. The sample will most always be returned with the results you desire.