How do I read and update a dataset in Form1 from Form2

I am a VB .NET novice, so here is an easy one, I hope?

I have a dataset which hold 2 tables - 1. Client details
2. ZipCode

I have a form (Form1)where I can edit a clients details. on that screen I have a combo box ( which contains info from Zipcode table in the dataset) where the user can choose the city. When the city is chosen the ZipCode is automatically entered from my dataset.

If the ZipCode is not in the dataset I want the user to be able to add it via Form2 which will open by clicking a button on Form1.

Can anyone tell me how I can read and update the dataset in Form1 from Form2?

Thankyou.