Absolutely. You can change your plan every month (to a bigger or smaller one) in the 'plan and add-ons' section of your account. If you've selected a new plan, it will start on your next renewal date. 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.
Please keep in mind that your unused data is yours to keep in your data bank for as long as you stay on your current plan or change to a bigger one. If you change your plan to a smaller one, your data bank will reset, and you will lose any of the data you had banked so far. From that point on, you can start banking any unused data again.