Billing managers can upgrade or downgrade a client's paid organization at any time.
If you're not a billing manager for the organization, ask your client to have an owner of the organization add you to the organization as a billing manager.
Tip
In the upper-right corner of any page on GitHub, click your profile photo, then click Settings.
In the "Access" section of the sidebar, click Organizations.
Next to the organization, click Settings.
If you are an organization owner, in the "Access" section of the sidebar, click Billing and plans.
Under "Current plan", next to the name of your plan, select Edit and click Add seats.
Under "Add seats", type the number of seats you'd like to add.
Click Add seats.
After you add seats, the payment method on file for the organization will be charged a pro-rated amount based on the number of seats you're adding and the amount of time left in your billing cycle.