Alter devise update method and strong params?

Question

When devise is working with a custom controller (users), how can we alter the update method to happily accept a change in some attributes without requiring password to be submitted as well?

E.g. the form won't let the user update their timezone without also entering their password (twice)

What I know / have tried

I managed to alter the create method, now I wish to alter the update method, and presumably the strong params. I can't locate the strong params and I may not be confident knowing how to alter them if I was successful in finding them

Notes

• updating the timezone (or any user attributes) results in a PUT to Users::RegistrationsController#update

• Users::RegistrationsController inherits from Devise::RegistrationsController, but I have no idea where to find Devise::RegistrationsController (I can see it on github but just not in my app)

• I tried adding this to Users::RegistrationsController but it didn't seem to help