Upgrading your plan:
To upgrade your plan, go to the Subscription section and click on the "Change" link:
Next, you can choose the plan that you wish to upgrade to:
How to enable overage credits:
You can enable overage credits from your Subscription page when you reach the plan quota.
These will be added to your next bill when your monthly plan renews.
To do that, you can click on Disable or edit limits option to enable credits over the quota
You can also enter the maximum number of search credits over quota per month or verify credits over quota per month: