After the free trial period expires
After the trial period expires, you will only be able to switch to the flexible plan when you renew your subscription.
Sign in to your Google Admin account. Sign in with an administrator account, not a user account. On the main page of the administration console, at the top left, click on Menu > Billing > Subscriptions. To the left of the relevant subscription, click the down arrow. Next to “Renewal Options”, click Edit. Select the Switch to flexible plan option. Click on Register.
The next time you renew your annual subscription, you will automatically switch to the flexible plan.