Can I change which 'Format' options are available in my Rich Text field? I want to update my Rich Text field so only H1, H2 and H3 are the available options in the 'Format' selector.

Is this possible?
You certainly can using the Redactor Formatting config.

Locate the config file for the Redactor setting you want to edit in craft/config/redactor and add this line:

"formatting": ["h1", "h2", "h3"],

Or any combination of elements you want to be made available.

For those reading this who can't see the Formatting option for their Redactor field, it's also worth mentioning you need to have the Formatting button set up in the same config file:

"buttons": ["format"],