A circle has a "name" and a "short name".  The short name is used on the Org Chart to save space. These are modified in different ways.

A circle's name is modified through Governance in its supercircle.   Go to the Circle's supercircle, open a Governance Meeting, and choose Add/Edit Role.   Since a Circle is a special kind of role, you modify the Circle by modifying the corresponding role in the supercircle. 

(Premium users may also use Asynchronous Governance Proposals to do the same thing.)

You can change the circle's "short name" by using the gear icon that appears after the circle's name on the circle page.  The short name is the one that appears on the Org Chart. Only the Lead Link, Secretary or an organization GlassFrog Admin can do this.  See screenshot: