Only a User who has Owner or Admin permissions in the master account can change your plan. You can upgrade or downgrade your plan from Settings > Plan. If using Subaccounts, you'll need to go into your master account before clicking Settings. You cannot edit your plan from within a Subaccount.
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 and your billing date will change to today's date. When you upgrade your plan, the new plan's entire credit allotment will go into your account immediately and you will keep all remaining credits from your previous plan.