Source: https://docs.github.com/en/billing/managing-billing-for-github-sponsors/downgrading-a-sponsorship
Canceling a sponsorship
- In the upper-right corner of any page, click your profile photo, then click Settings.
- In your user settings sidebar, click Billing & plans.
- Under "GitHub Sponsors", to the right of the sponsored open source contributor, click  next to your sponsored amount, then click Cancel sponsorship.
- Review the information about canceling your sponsorship, then click OK.


Optionally, to manage your sponsorship on behalf of an organization, in the upper-left corner, use the Personal settings drop-down menu, and click the organization.


