To make a new field in Salesforce visible (or not visible) to Marketo, you will need to make changes to the field's visibility on the Salesforce profile that is used by the "Marketo Sync User" Some instructions on how to edit field visibily in SFDC here: https://community.marketo.com/MarketoArticle?id=kA050000000LJOjCAO
I'm not sure what you mean by updating fields in Salesforce, but if you were to add a new value to a picklist, it would sync to Marketo. You may need to reapprove any landing pages/forms that use that field in order to make the new value visible.
Thanks so much Val!
I think this is exactly what I was looking for. :)
Looks like it has changed a little bit in SalesForce since that article came out. Here is an updated article on where to find the Fields in SF.