If you've selected a new plan, it will start on your next renewal date. You can find your plan renewal date in the 'plan and add-ons' section of your account. Please make sure to change your plan before 8pm the day before your renewal date for your new plan to start on your next plan renewal date. If you change your plan after 8pm, your new plan may not start until the month after.
If you need more data before your renewal date, you can buy extra data in the same section of your account.