When I go to Admin > Field Management, I can't find a way to change the field mappings or to map fields that are currently un-mapped.
I don't see the 'Edit Mappings' tab that Marketo shows here: Edit Initial Field Mappings - Marketo Docs - Product Docs.
You can only do this on the initial setup.
What you see there are orphan fields and this is because those aren't mapped to other objects. I suspect if you looked at the first group, it would be Lead>Contact or something.
The fields are available, however, in SFDC they aren't mapped to each other.
But in SFDC, you can modify the mappings. Unfortunately, if you do this after the sync, Marketo will have issues, so you need Support to help. There are threads on this, but the process is