A circle is a special kind of role from the super circle's point of view. Therefore, to change the purpose of a circle, you will need to open a Governance Meeting of its super circle. From there, you can edit the sub circle and modify the purpose.
Note: Your organization's purpose is the purpose of its largest circle. See the following link to see how to change the purpose of the organization.
In the scenario below, you're changing the purpose of the Market Place's circle. Since Market Place is a sub circle of GCC, you will be opening a Governance meeting for the GCC circle.
Then you can edit the purpose as shown below, and Click the "Accept Proposal" button at the bottom of the screen:
Great job! You're all done!