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.