Update fields pre save

Hi everyone

I've been struggling with a problem I'm sure it's easy. On cloning a potential I want a button that will empty some of the fields.. Before saving.

I can create a button, and tried a bunch of ways in function to set fields on pressing it. Any tips please? What would be the syntax to set the "Stage" field in the "Potentials" module to "-None-" . I would then repeat the example to other fields and also try to update a date to be there cloned dates value plus 365 days:)

Or... Can you tweak how the Clone button works at all?

Thanks
Jim