You can upgrade or downgrade your plan from Settings > Plan. However, only a user who has owner or admin permissions in the master account can access plans and billing.
If you're using subaccounts, you'll need to log in to your master account to make any changes.
Plan upgrades take effect immediately. When you upgrade your plan in the middle of a billing period, you will be charged the full amount of the new plan, your billing date will change to today's date, and the new plan's entire credit allotment will go into your account immediately. You will keep all remaining credits from your previous plan.