How to access

This idea seems to go against the principle of encapsulation.
A better approach would be for the first ViewController to make a change to a shared Model, which the second ViewController can then respond to.

You don't tell what you want to do precisely.

But if you really need and cannot use robnotyou solution,

• get an instance vc2 of VC2 in the first VC VC1
• call vc2.button in VC1

Or use delegation.