Not sure it functions like this but it would be a great option if not. Could someone let me know about it?
So I am now using the styles panel to setup reusable styles.
What I would like to know is if it is then possible to create multiple elements (say buttons on several screens) apply the style to them, then simply edit my style definition and have all buttons update automatically?
Or do I actually need to manual select each button and apply the new style?
No, that is not possible. Styles in Photoshop are not live styles - they are presets (things you create once and reuse).
You would have to select each and apply the new style.