It requires diamonds..  to do so though, you'll access it via going to your alliance page.  Then click on 'settings' at the bottom.. and there will be an 'expand' option.

EF's Princess said:

I have the diamonds but I'm only a marshal ... 

Do you mean just a general?  ... Marshall is the highest rank, as in the leader..  Only the Marshall may do these tasks as they are the only ones currently able to change settings of the alliance..